feat(projects): Tidy page for fuzzy-detecting + bulk-merging duplicate projects

Phase 5b first slice.  Surfaces near-duplicate projects (typo variants,
abbreviation differences, spacing variations like "SR81" vs "SR 81")
as side-by-side pairs the operator can merge with one click.

Backend (backend/services/project_tidy.py):
- find_duplicate_pairs(db, threshold=0.85) walks all active projects and
  computes rapidfuzz.WRatio similarity for every pair.  Pre-filters
  too-short normalised names (< 4 chars) to avoid noise.  Skips
  soft-deleted projects.  Returns pairs sorted by score desc, then by
  total content (more assignments → review first).
- Each pair carries a suggested merge target with a human-readable
  reason.  Priorities (in order): manual source over parser source,
  populated project_number, more locations, more assignments, shorter
  name.  Operator can override the suggestion by clicking the OTHER
  direction button.
- O(N^2) over project count.  Fine up to ~500 projects.  Token-prefix
  blocking is the obvious next optimisation if it becomes slow.

Backend (backend/routers/projects.py):
- GET /api/projects/admin/duplicate_pairs?threshold=&max_pairs=  returns
  pairs as JSON for the Tidy page.

Frontend (templates/admin/project_tidy.html):
- New admin page at /settings/developer/project-tidy.  Threshold selector
  (95% / 90% / 85% / 80%) at the top; rescan button next to it; auto-
  scans on load.
- Each pair card shows side-by-side project summaries (name, project_
  number, client, source-badge, location/assignment counts) with the
  suggested target visually highlighted (orange border).  Three buttons:
  "Merge A → B", "Merge B → A", "Not a dup" (hide locally).
- Click-to-merge opens a native confirm with the preview totals
  (assignments/sessions/data files moving, consolidations) — same data
  the project_header.html merge modal shows.  On confirm, hits the
  existing /merge_into endpoint and re-scans automatically.
- Source badges distinguish parser-created (`metadata_backfill`) from
  manual projects — at a glance the operator can see "this duplicate is
  parser-generated; safe to merge into the manual one".

Frontend (templates/admin/metadata_backfill.html):
- Apply-result handling now surfaces failed[] cluster reasons in a
  dedicated failure panel (bottom-left, dismissable).  Previously a 200
  OK with all-failures showed a misleading "1 cluster applied" success
  toast because the count and the failure list weren't being reconciled.
  This bit us during the DB-revert recovery earlier — the
  project_modules table was missing, every apply silently rolled back,
  user saw success toasts.  Fixed.

Smoke-verified against current state (10K events, 9 projects, post-
merge): tool correctly finds 0 pairs at threshold 0.85 (data is clean),
1 false-positive at 0.70 (two unrelated projects sharing the token "81"
— example of why the 0.85 default is correct).

Settings link added under Developer → Project Tidy.

Phase 5c (swap-detection daily background job + notification inbox)
remains deferred to the next session.

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

templates/settings.html

@@ -574,6 +574,20 @@
                         Open
                     </a>
                 </div>
+
+                <!-- Project Tidy (Phase 5b) -->
+                <div class="flex items-center justify-between p-4 bg-gray-50 dark:bg-slate-700 rounded-lg">
+                    <div>
+                        <div class="font-medium text-gray-900 dark:text-white">Project Tidy</div>
+                        <div class="text-sm text-gray-500 dark:text-gray-400 mt-0.5">
+                            Find duplicate-looking projects via fuzzy name match (typos, abbreviations, spacing variations) and bulk-merge them.
+                        </div>
+                    </div>
+                    <a href="/settings/developer/project-tidy"
+                       class="ml-6 px-4 py-2 bg-seismo-orange hover:bg-orange-600 text-white text-sm font-medium rounded-lg transition-colors whitespace-nowrap">
+                        Open
+                    </a>
+                </div>
             </div>
         </div>
     </div>