feat(sfm): editable UnitAssignment date windows (backdate deployments)

Operators couldn't change a unit's assigned_at / assigned_until after
creating the assignment, so a unit physically deployed in December 2025
but only recorded in terra-view today would show "deployed today" and
all its real events would be invisible on the project's location page.

Backend:
- PATCH /api/projects/{project_id}/assignments/{assignment_id}
  Accepts JSON body with optional assigned_at, assigned_until, notes.
  - assigned_at is required (cannot be cleared)
  - assigned_until can be null to mark active / indefinite
  - assigned_until must be after assigned_at
  - rejects overlaps with other assignments of the same unit at the
    same location (different units overlapping is fine — that's a
    legitimate swap window)
  - assignment.status flips to "active" when assigned_until is cleared,
    "completed" when set
  - 404 if the assignment doesn't belong to {project_id} (security)

Frontend (vibration_location_detail.html):
- Pencil icon next to each row in the "Seismographs deployed at this
  location" card. Click to open a modal with datetime-local inputs for
  From + Until (blank = active) and a Notes textarea. Save reloads the
  Events tab so KPI tiles and the event table reflect the new window.
- Helper line under the assignment list explains the workflow:
  "Click the pencil to backdate a deployment so historical events get
  attributed to this location."

Verified end-to-end against real data: backdating BE11529's assignment
on a vibration location from 2026-04-14 to 2025-12-01 surfaced 10
additional events (24 -> 34) that were previously invisible.

Validation suite (all returning correct HTTP codes):
  - assigned_until < assigned_at -> 400
  - cross-project assignment_id -> 404
  - assigned_at cleared -> 400
  - notes-only update -> 200

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

templates/vibration_location_detail.html

@@ -220,6 +220,66 @@
                 <span id="ev-assignments-count" class="text-xs text-gray-500 dark:text-gray-400"></span>
             </div>
             <div id="ev-assignments-list" class="divide-y divide-gray-200 dark:divide-gray-700"></div>
+            <p class="text-xs text-gray-500 dark:text-gray-400 mt-3">
+                <span class="inline-block w-4 text-center">✎</span>
+                Click the pencil to backdate a deployment so historical events get attributed to this location.
+            </p>
+        </div>
+
+        <!-- Edit-assignment modal -->
+        <div id="assignment-edit-modal" class="hidden fixed inset-0 bg-black bg-opacity-50 z-50 flex items-center justify-center">
+            <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-md m-4">
+                <div class="p-6 border-b border-gray-200 dark:border-gray-700 flex items-center justify-between">
+                    <div>
+                        <h2 class="text-xl font-bold text-gray-900 dark:text-white">Edit Deployment Window</h2>
+                        <p class="text-sm text-gray-500 dark:text-gray-400 mt-1">
+                            <span id="ae-unit-label" class="font-mono text-seismo-orange">—</span>
+                        </p>
+                    </div>
+                    <button onclick="closeAssignmentEditModal()" class="text-gray-500 hover:text-gray-700 dark:text-gray-400">
+                        <svg class="w-6 h-6" 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"></path>
+                        </svg>
+                    </button>
+                </div>
+                <form id="assignment-edit-form" class="p-6 space-y-4">
+                    <input type="hidden" id="ae-assignment-id">
+
+                    <div>
+                        <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Assigned From</label>
+                        <input type="datetime-local" id="ae-assigned-at" required
+                               class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white">
+                    </div>
+
+                    <div>
+                        <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
+                            Assigned Until
+                            <span class="text-xs text-gray-500 ml-1">(leave blank if still active)</span>
+                        </label>
+                        <input type="datetime-local" id="ae-assigned-until"
+                               class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white">
+                    </div>
+
+                    <div>
+                        <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">Notes</label>
+                        <textarea id="ae-notes" rows="2" placeholder="Optional — e.g. 'backdated to reflect physical install date'"
+                                  class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white"></textarea>
+                    </div>
+
+                    <div id="ae-error" class="hidden text-sm text-red-600"></div>
+
+                    <div class="flex justify-end gap-3 pt-2">
+                        <button type="button" onclick="closeAssignmentEditModal()"
+                                class="px-4 py-2 border border-gray-300 dark:border-gray-600 rounded-lg text-gray-700 dark:text-gray-300 hover:bg-gray-50 dark:hover:bg-gray-700">
+                            Cancel
+                        </button>
+                        <button type="submit" id="ae-submit-btn"
+                                class="px-4 py-2 bg-seismo-orange hover:bg-seismo-navy text-white rounded-lg font-medium">
+                            Save
+                        </button>
+                    </div>
+                </form>
+            </div>
         </div>
 
         <!-- Filters -->
@@ -504,17 +564,100 @@ function renderAssignmentsUsed(assignments) {
         const badge = isActive
             ? '<span class="ml-2 px-2 py-0.5 rounded text-xs bg-green-100 text-green-800 dark:bg-green-900/30 dark:text-green-300">active</span>'
             : '';
-        return `<div class="py-2 flex items-center justify-between">
-            <div>
+        const editAttr = encodeURIComponent(JSON.stringify({
+            id: a.assignment_id,
+            unit_id: a.unit_id,
+            assigned_at: a.assigned_at,
+            assigned_until: a.assigned_until,
+        }));
+        return `<div class="py-2 flex items-center justify-between gap-3">
+            <div class="flex items-center gap-2 min-w-0">
                 <a href="/unit/${esc(a.unit_id)}" class="font-mono font-semibold text-seismo-orange hover:text-seismo-navy">${esc(a.unit_id)}</a>
                 ${badge}
-                <span class="ml-3 text-sm text-gray-600 dark:text-gray-400">${start} → ${end}</span>
+                <span class="text-sm text-gray-600 dark:text-gray-400">${start} → ${end}</span>
+                <button type="button"
+                        onclick="openAssignmentEditModal('${editAttr}')"
+                        title="Edit deployment dates"
+                        class="text-gray-400 hover:text-seismo-orange transition-colors p-1">
+                    <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M11 5H6a2 2 0 00-2 2v11a2 2 0 002 2h11a2 2 0 002-2v-5m-1.414-9.414a2 2 0 112.828 2.828L11.828 15H9v-2.828l8.586-8.586z"></path>
+                    </svg>
+                </button>
             </div>
-            <span class="text-sm text-gray-700 dark:text-gray-300">${(a.events_in_window || 0).toLocaleString()} event${a.events_in_window === 1 ? '' : 's'}</span>
+            <span class="text-sm text-gray-700 dark:text-gray-300 whitespace-nowrap">${(a.events_in_window || 0).toLocaleString()} event${a.events_in_window === 1 ? '' : 's'}</span>
         </div>`;
     }).join('');
 }
 
+// ── Assignment-edit modal ───────────────────────────────────────────────────
+function _isoToInputValue(iso) {
+    // Convert "2026-04-14T02:19:27" (or "2026-04-14 02:19:27") to "2026-04-14T02:19" for datetime-local input.
+    if (!iso) return '';
+    const cleaned = iso.replace(' ', 'T');
+    return cleaned.slice(0, 16);
+}
+
+function openAssignmentEditModal(encodedJson) {
+    const data = JSON.parse(decodeURIComponent(encodedJson));
+    document.getElementById('ae-assignment-id').value = data.id;
+    document.getElementById('ae-unit-label').textContent = data.unit_id;
+    document.getElementById('ae-assigned-at').value    = _isoToInputValue(data.assigned_at);
+    document.getElementById('ae-assigned-until').value = _isoToInputValue(data.assigned_until);
+    document.getElementById('ae-notes').value = '';
+    document.getElementById('ae-error').classList.add('hidden');
+    document.getElementById('assignment-edit-modal').classList.remove('hidden');
+}
+
+function closeAssignmentEditModal() {
+    document.getElementById('assignment-edit-modal').classList.add('hidden');
+}
+
+document.getElementById('assignment-edit-form').addEventListener('submit', async function(e) {
+    e.preventDefault();
+    const errEl = document.getElementById('ae-error');
+    errEl.classList.add('hidden');
+
+    const assignmentId = document.getElementById('ae-assignment-id').value;
+    const assignedAt    = document.getElementById('ae-assigned-at').value;
+    const assignedUntil = document.getElementById('ae-assigned-until').value;
+    const notes         = document.getElementById('ae-notes').value.trim();
+
+    if (!assignedAt) {
+        errEl.textContent = 'Assigned From is required.';
+        errEl.classList.remove('hidden');
+        return;
+    }
+
+    const payload = { assigned_at: assignedAt };
+    payload.assigned_until = assignedUntil || null;
+    if (notes) payload.notes = notes;
+
+    const btn = document.getElementById('ae-submit-btn');
+    btn.disabled = true; btn.textContent = 'Saving…';
+    try {
+        const r = await fetch(`/api/projects/${projectId}/assignments/${assignmentId}`, {
+            method:  'PATCH',
+            headers: {'Content-Type': 'application/json'},
+            body:    JSON.stringify(payload),
+        });
+        if (!r.ok) {
+            const err = await r.json().catch(() => ({detail: 'HTTP ' + r.status}));
+            throw new Error(err.detail || 'HTTP ' + r.status);
+        }
+        closeAssignmentEditModal();
+        await loadLocationEvents();   // Refresh stats + table with new window.
+    } catch (err) {
+        errEl.textContent = err.message || 'Failed to update assignment.';
+        errEl.classList.remove('hidden');
+    } finally {
+        btn.disabled = false; btn.textContent = 'Save';
+    }
+});
+
+document.getElementById('assignment-edit-modal').addEventListener('click', function(e) {
+    if (e.target === this) closeAssignmentEditModal();
+});
+
 function renderEventTable(events, total, container) {
     if (!events || events.length === 0) {
         const haveAssignments = !document.getElementById('events-assignments-card').classList.contains('hidden');