feat(projects): "Merge into…" button to consolidate duplicate projects

Operator-facing tool for cleaning up duplicate projects.  Common after
the metadata-backfill parser auto-creates near-duplicates from operator
name variations ("SR81" vs "SR 81", "Swank-Karns Crossing" vs
"Swank-Karns Crossings", "Trumbull-Bryman Mont.Dam" vs
"Trumbull-Brayman-Mont Dam", etc.).

Workflow: visit the duplicate project's detail page, click "Merge into…"
in the header, search for the canonical target project from a typeahead,
review the preview (what assignments / locations / sessions will move,
any conflicts), confirm.  Source is soft-deleted; everything else
re-points to the target.  Smart consolidation: same-named locations in
both projects merge into one (source's assignments move to target's
existing location with the same name; source's empty location is then
deleted).  Different-named locations move as-is.

Backend:
- backend/services/project_merge.py (new): preview() and execute()
  functions.  Transaction-safe.  Per-assignment UnitHistory audit row
  with change_type='assignment_merged' so the deployment timeline shows
  the merge.  Source modules disabled; missing modules added to target.
  Handles edge cases: same project_id rejected, deleted projects rejected,
  orphan project-direct assignments (no location) re-pointed defensively.

- backend/routers/projects.py: new endpoints
    GET  /api/projects/{source_id}/merge_preview?target_id=...
    POST /api/projects/{source_id}/merge_into?target_id=...

Frontend (templates/partials/projects/project_header.html):
- "Merge into…" button in Project Actions area.
- Modal with typeahead (reuses /api/admin/metadata_backfill/projects_search)
  scoped to existing projects only (no create-new option).  Filters out
  the source project from candidates so operator can't accidentally pick
  it as target.
- Preview pane shows totals + per-location plan (consolidate vs move) +
  warnings (mismatched client names, location consolidation note).
- Red "Merge (permanent)" confirm button only enables after a target is
  picked and preview loads.
- On success, browser redirects to target project page.

Smoke verified: "Swank-Karns Crossing" (1 assignment) merged into
"Swank-Karns Crossings"; target now has 2 locations + 2 assignments,
source has 0 dangling rows, 1 project_merge audit entry written.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

templates/partials/projects/project_header.html

@@ -75,10 +75,266 @@
                 Generate Combined Report
             </a>
             {% endif %}
+            <button onclick="openMergeModal()"
+                    title="Merge this project into another (consolidates duplicates)"
+                    class="px-4 py-2 border border-gray-300 dark:border-gray-600 text-gray-700 dark:text-gray-300 rounded-lg hover:bg-gray-50 dark:hover:bg-gray-700 transition-colors flex items-center gap-2 text-sm">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M17 8l4 4m0 0l-4 4m4-4H3"></path>
+                </svg>
+                Merge into…
+            </button>
         </div>
     </div>
 </div>
 
+<!-- Merge Modal -->
+<div id="merge-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm">
+    <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-2xl mx-4 max-h-[90vh] flex flex-col">
+        <!-- Header -->
+        <div class="px-6 py-4 border-b border-gray-200 dark:border-gray-700 flex items-center justify-between">
+            <div>
+                <h3 class="text-lg font-semibold text-gray-900 dark:text-white">Merge "{{ project.name }}" into another project</h3>
+                <p class="text-sm text-gray-500 dark:text-gray-400 mt-0.5">
+                    Source project gets soft-deleted.  All its locations, assignments, sessions, and files move to the target.
+                </p>
+            </div>
+            <button onclick="closeMergeModal()" class="text-gray-400 hover:text-gray-600 dark:hover:text-gray-200 shrink-0 ml-3">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/></svg>
+            </button>
+        </div>
+
+        <!-- Body -->
+        <div class="px-6 py-4 overflow-y-auto flex-1">
+            <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
+                Target project
+            </label>
+            <div class="relative">
+                <input type="text"
+                       id="merge-target-input"
+                       placeholder="Type to search for the project to merge INTO…"
+                       autocomplete="off"
+                       oninput="onMergeTargetInput()"
+                       onfocus="onMergeTargetInput()"
+                       onblur="setTimeout(() => document.getElementById('merge-target-dropdown').classList.add('hidden'), 150)"
+                       class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-slate-700 text-gray-900 dark:text-white text-sm">
+                <input type="hidden" id="merge-target-id" value="">
+                <div id="merge-target-dropdown"
+                     class="hidden absolute z-20 left-0 right-0 mt-1 bg-white dark:bg-slate-800 border border-gray-200 dark:border-gray-700 rounded-lg shadow-lg max-h-64 overflow-y-auto"></div>
+            </div>
+
+            <!-- Preview pane -->
+            <div id="merge-preview" class="mt-4 hidden">
+                <h4 class="text-xs font-semibold text-gray-500 dark:text-gray-400 uppercase tracking-wider mb-2">What will move</h4>
+                <div id="merge-preview-body" class="space-y-3"></div>
+            </div>
+
+            <div id="merge-error" class="hidden mt-3 text-sm text-red-600"></div>
+        </div>
+
+        <!-- Footer -->
+        <div class="px-6 py-4 border-t border-gray-200 dark:border-gray-700 flex justify-end gap-2">
+            <button onclick="closeMergeModal()"
+                    class="px-4 py-2 border border-gray-300 dark:border-gray-600 text-gray-700 dark:text-gray-300 rounded-lg hover:bg-gray-50 dark:hover:bg-gray-700 text-sm">
+                Cancel
+            </button>
+            <button id="merge-confirm-btn"
+                    onclick="confirmMerge()"
+                    disabled
+                    class="px-4 py-2 bg-red-600 hover:bg-red-700 text-white rounded-lg text-sm font-medium disabled:opacity-40 disabled:cursor-not-allowed">
+                Merge (permanent)
+            </button>
+        </div>
+    </div>
+</div>
+
+<script>
+const _SOURCE_PROJECT_ID   = "{{ project.id }}";
+const _SOURCE_PROJECT_NAME = {{ project.name|tojson }};
+
+function _mergeEsc(s) {
+    if (s == null) return '';
+    return String(s).replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
+}
+
+function openMergeModal() {
+    document.getElementById('merge-target-input').value = '';
+    document.getElementById('merge-target-id').value = '';
+    document.getElementById('merge-preview').classList.add('hidden');
+    document.getElementById('merge-error').classList.add('hidden');
+    document.getElementById('merge-confirm-btn').disabled = true;
+    document.getElementById('merge-modal').classList.remove('hidden');
+    setTimeout(() => document.getElementById('merge-target-input').focus(), 50);
+}
+
+function closeMergeModal() {
+    document.getElementById('merge-modal').classList.add('hidden');
+}
+
+let _mergeTargetDebounce = null;
+async function onMergeTargetInput() {
+    if (_mergeTargetDebounce) clearTimeout(_mergeTargetDebounce);
+    _mergeTargetDebounce = setTimeout(_mergeFetchTargets, 150);
+}
+
+async function _mergeFetchTargets() {
+    const q = document.getElementById('merge-target-input').value.trim();
+    const dropdown = document.getElementById('merge-target-dropdown');
+
+    let data;
+    try {
+        // Reuse the metadata-backfill projects_search endpoint — works for
+        // any caller, returns existing-only (no create-new option needed here).
+        const r = await fetch(`/api/admin/metadata_backfill/projects_search?q=${encodeURIComponent(q)}&limit=12`);
+        if (!r.ok) throw new Error('HTTP ' + r.status);
+        data = await r.json();
+    } catch (e) {
+        dropdown.innerHTML = `<div class="px-3 py-2 text-sm text-red-500">Search failed: ${_mergeEsc(e.message)}</div>`;
+        dropdown.classList.remove('hidden');
+        return;
+    }
+
+    // Filter out self.
+    const candidates = (data.matches || []).filter(m => m.id !== _SOURCE_PROJECT_ID);
+
+    if (candidates.length === 0) {
+        dropdown.innerHTML = `<div class="px-3 py-2 text-sm text-gray-500 dark:text-gray-400 italic">No matches.</div>`;
+        dropdown.classList.remove('hidden');
+        return;
+    }
+
+    dropdown.innerHTML = candidates.map(m => {
+        const scoreBadge = m.score >= 0.99
+            ? '<span class="text-xs text-green-600 dark:text-green-400 ml-2">exact</span>'
+            : `<span class="text-xs text-gray-500 dark:text-gray-400 ml-2">${(m.score*100).toFixed(0)}%</span>`;
+        const meta = [];
+        if (m.project_number) meta.push(_mergeEsc(m.project_number));
+        if (m.client_name)    meta.push(_mergeEsc(m.client_name));
+        if (m.location_count > 0) meta.push(`${m.location_count} location${m.location_count === 1 ? '' : 's'}`);
+        const metaLine = meta.length ? `<div class="text-xs text-gray-500 dark:text-gray-400">${meta.join(' · ')}</div>` : '';
+        return `<button type="button"
+            onmousedown="event.preventDefault()"
+            onclick="onMergePickTarget('${_mergeEsc(m.id)}', ${JSON.stringify(m.name)})"
+            class="w-full text-left px-3 py-2 hover:bg-gray-50 dark:hover:bg-slate-700 border-b border-gray-100 dark:border-gray-700 last:border-b-0">
+            <div class="text-sm font-medium text-gray-900 dark:text-white">${_mergeEsc(m.name)}${scoreBadge}</div>
+            ${metaLine}
+        </button>`;
+    }).join('');
+    dropdown.classList.remove('hidden');
+}
+
+async function onMergePickTarget(targetId, targetName) {
+    document.getElementById('merge-target-input').value = targetName;
+    document.getElementById('merge-target-id').value = targetId;
+    document.getElementById('merge-target-dropdown').classList.add('hidden');
+    await _loadMergePreview(targetId);
+}
+
+async function _loadMergePreview(targetId) {
+    const previewEl = document.getElementById('merge-preview');
+    const bodyEl    = document.getElementById('merge-preview-body');
+    const errorEl   = document.getElementById('merge-error');
+    const confirmBtn = document.getElementById('merge-confirm-btn');
+
+    previewEl.classList.add('hidden');
+    errorEl.classList.add('hidden');
+    confirmBtn.disabled = true;
+    bodyEl.innerHTML = '<div class="text-center py-3 text-sm text-gray-500"><div class="animate-spin rounded-full h-5 w-5 border-b-2 border-seismo-orange mx-auto mb-2"></div>Loading preview…</div>';
+    previewEl.classList.remove('hidden');
+
+    let d;
+    try {
+        const r = await fetch(`/api/projects/${_SOURCE_PROJECT_ID}/merge_preview?target_id=${encodeURIComponent(targetId)}`);
+        if (!r.ok) {
+            const err = await r.json().catch(() => ({detail: 'HTTP ' + r.status}));
+            throw new Error(err.detail || ('HTTP ' + r.status));
+        }
+        d = await r.json();
+    } catch (e) {
+        errorEl.textContent = e.message;
+        errorEl.classList.remove('hidden');
+        previewEl.classList.add('hidden');
+        return;
+    }
+
+    let html = `<div class="text-sm text-gray-700 dark:text-gray-300">
+        Merging <strong>"${_mergeEsc(d.source_project_name)}"</strong> into <strong>"${_mergeEsc(d.target_project_name)}"</strong>:
+    </div>
+    <div class="grid grid-cols-3 gap-2 mt-2">
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Assignments</div><div class="text-lg font-bold">${d.total_assignments_moving}</div></div>
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Sessions</div><div class="text-lg font-bold">${d.total_sessions_moving}</div></div>
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Data files</div><div class="text-lg font-bold">${d.total_data_files_moving}</div></div>
+    </div>`;
+
+    if (d.location_plans.length > 0) {
+        html += `<div class="mt-3">
+            <h5 class="text-xs font-semibold text-gray-500 dark:text-gray-400 uppercase tracking-wider mb-1">Locations</h5>
+            <div class="space-y-1 text-sm">`;
+        for (const p of d.location_plans) {
+            if (p.action === 'consolidate') {
+                html += `<div class="text-gray-700 dark:text-gray-300">
+                    🔀 <strong>${_mergeEsc(p.source_name)}</strong> → consolidates into existing target <strong>"${_mergeEsc(p.target_name)}"</strong>
+                    <span class="text-xs text-gray-500 dark:text-gray-400 ml-1">(${p.assignments_moving} assignments + ${p.sessions_moving} sessions move)</span>
+                </div>`;
+            } else {
+                html += `<div class="text-gray-700 dark:text-gray-300">
+                    → <strong>${_mergeEsc(p.source_name)}</strong> moves to target as-is
+                    <span class="text-xs text-gray-500 dark:text-gray-400 ml-1">(${p.assignments_moving} assignments + ${p.sessions_moving} sessions)</span>
+                </div>`;
+            }
+        }
+        html += '</div></div>';
+    }
+
+    if (d.modules_to_add.length > 0) {
+        html += `<div class="mt-3 text-sm text-gray-700 dark:text-gray-300">
+            Modules to add to target: ${d.modules_to_add.map(m => `<code class="px-1 py-0.5 rounded bg-gray-100 dark:bg-gray-700 text-xs">${_mergeEsc(m)}</code>`).join(' ')}
+        </div>`;
+    }
+
+    if (d.warnings.length > 0) {
+        html += '<div class="mt-3 space-y-2">';
+        for (const w of d.warnings) {
+            html += `<div class="bg-amber-50 dark:bg-amber-900/20 border border-amber-200 dark:border-amber-800 rounded p-2 text-xs text-amber-800 dark:text-amber-300">⚠ ${_mergeEsc(w)}</div>`;
+        }
+        html += '</div>';
+    }
+
+    html += `<div class="mt-3 bg-red-50 dark:bg-red-900/20 border border-red-200 dark:border-red-800 rounded p-2 text-xs text-red-800 dark:text-red-300">
+        <strong>⚠ This action is destructive.</strong>  Source project will be soft-deleted (status='deleted').
+        Audit rows will be written to unit_history for every moved assignment.
+    </div>`;
+
+    bodyEl.innerHTML = html;
+    confirmBtn.disabled = false;
+}
+
+async function confirmMerge() {
+    const targetId = document.getElementById('merge-target-id').value;
+    if (!targetId) return;
+    const confirmBtn = document.getElementById('merge-confirm-btn');
+    confirmBtn.disabled = true;
+    confirmBtn.textContent = 'Merging…';
+    try {
+        const r = await fetch(`/api/projects/${_SOURCE_PROJECT_ID}/merge_into?target_id=${encodeURIComponent(targetId)}`, {
+            method: 'POST',
+        });
+        if (!r.ok) {
+            const err = await r.json().catch(() => ({detail: 'HTTP ' + r.status}));
+            throw new Error(err.detail || ('HTTP ' + r.status));
+        }
+        const d = await r.json();
+        // Redirect to the target project — source no longer exists in active state.
+        window.location.href = `/projects/${d.target_project_id}`;
+    } catch (e) {
+        const errorEl = document.getElementById('merge-error');
+        errorEl.textContent = 'Merge failed: ' + e.message;
+        errorEl.classList.remove('hidden');
+        confirmBtn.disabled = false;
+        confirmBtn.textContent = 'Merge (permanent)';
+    }
+}
+</script>
+
 <!-- Add Module Modal -->
 <div id="add-module-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm">
     <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-sm mx-4 p-6">