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>

backend/routers/projects.py

@@ -688,6 +688,72 @@ async def restore_project(project_id: str, db: Session = Depends(get_db)):
     return {"success": True, "message": f"Project '{project.name}' restored."}
 
 
+# ── Project merge ──────────────────────────────────────────────────────────────
+# Consolidate a duplicate project into another.  Common after the
+# metadata-backfill parser creates near-duplicate projects from name
+# variations operators typed on the BW device.
+# See backend/services/project_merge.py for the merge logic.
+
+@router.get("/{source_id}/merge_preview")
+async def project_merge_preview(
+    source_id: str,
+    target_id: str,
+    db: Session = Depends(get_db),
+):
+    """Preview what the merge will do — used by the confirmation modal.
+    No writes."""
+    from backend.services import project_merge as pm
+    preview = pm.preview(db, source_id, target_id)
+    return {
+        "source_project_id":         preview.source_project_id,
+        "source_project_name":       preview.source_project_name,
+        "target_project_id":         preview.target_project_id,
+        "target_project_name":       preview.target_project_name,
+        "total_assignments_moving":  preview.total_assignments_moving,
+        "total_sessions_moving":     preview.total_sessions_moving,
+        "total_data_files_moving":   preview.total_data_files_moving,
+        "modules_to_add":            preview.modules_to_add,
+        "warnings":                  preview.warnings,
+        "location_plans": [
+            {
+                "source_id":          p.source_id,
+                "source_name":        p.source_name,
+                "target_id":          p.target_id,
+                "target_name":        p.target_name,
+                "action":             p.action,
+                "assignments_moving": p.assignments_moving,
+                "sessions_moving":    p.sessions_moving,
+            }
+            for p in preview.location_plans
+        ],
+    }
+
+
+@router.post("/{source_id}/merge_into")
+async def project_merge_execute(
+    source_id: str,
+    target_id: str,
+    db: Session = Depends(get_db),
+):
+    """Execute the merge.  Source project gets soft-deleted; all its
+    locations / assignments / sessions / data_files / modules move to
+    the target.  Same-named locations consolidate."""
+    from backend.services import project_merge as pm
+    result = pm.execute(db, source_id, target_id, decided_by="operator")
+    return {
+        "success":                True,
+        "source_project_id":      result.source_project_id,
+        "target_project_id":      result.target_project_id,
+        "assignments_moved":      result.assignments_moved,
+        "locations_moved":        result.locations_moved,
+        "locations_consolidated": result.locations_consolidated,
+        "sessions_moved":         result.sessions_moved,
+        "data_files_moved":       result.data_files_moved,
+        "modules_added":          result.modules_added,
+        "audit_rows_written":     result.audit_rows_written,
+    }
+
+
 @router.get("/{project_id}")
 async def get_project(project_id: str, db: Session = Depends(get_db)):
     """