feat: ML tag suggestions, character/fandom integrity, underscores, modal polish

Consolidated merge of feat/tag-suggestions branch. Original 64-commit history
was lost to git-object corruption in a Nextcloud-synced checkout; this single
commit captures the equivalent diff.

Includes:
- pgvector-backed tag suggestion infra (WD14 + SigLIP centroids, ml-worker
  container, Celery tasks, suggestion service, accept/reject endpoints + modal
  UI with green/red chip buttons)
- Character/fandom integrity: title-case normalization on every write path,
  fandom-id backfill, maintenance task + settings button, migrations g26041901
  + h26041901 to canonicalize legacy rows with case-only duplicate merging
- Tag-underscores + modal polish: WD14 name canonicalization at emit + accept
  + add/bulk-add paths, migration i26041901 for legacy-row rename-or-merge
  across character/fandom/NULL kinds, suggestion-accept refresh parity via
  awaited loadTags, persistent chip tint

app/templates/settings.html

@@ -436,6 +436,35 @@
         </p>
       </div>
     </div>
+
+    <section class="settings-section">
+      <h3>ML tag suggestions</h3>
+      <p class="settings-hint">
+        Run inference on all images missing predictions or embeddings for the current model versions.
+        Resumable and safe to re-run. Backfill enqueues work on the <code>ml</code> queue; monitor progress in the ml-worker logs.
+      </p>
+      <form action="{{ url_for('main.trigger_ml_backfill') }}" method="post" onsubmit="return confirm('Enqueue ML backfill for all unprocessed images?');">
+        <button type="submit" class="btn secondary-btn">Run ML backfill</button>
+      </form>
+
+      <p class="settings-hint" style="margin-top: 1em;">
+        Recompute centroids for all eligible tags (character, fandom, and general/topic) that have at least
+        the minimum number of reference images. Run this after the initial backfill, or any time you've
+        applied many tags manually and want the suggestions to catch up.
+      </p>
+      <form action="{{ url_for('main.trigger_recompute_all_centroids') }}" method="post" onsubmit="return confirm('Enqueue centroid recompute for all eligible tags?');">
+        <button type="submit" class="btn secondary-btn">Recompute all centroids</button>
+      </form>
+
+      <p class="settings-hint" style="margin-top: 1em;">
+        Attach each character's fandom to every image already tagged with that character.
+        Additive only — existing fandom tags are never removed. Run after you've set a
+        fandom on an existing character, or whenever you suspect drift.
+      </p>
+      <form action="{{ url_for('main.trigger_sync_character_fandoms') }}" method="post" onsubmit="return confirm('Enqueue character↔fandom sync for all tagged images?');">
+        <button type="submit" class="btn secondary-btn">Sync character fandoms to images</button>
+      </form>
+    </section>
   </div>
 </div>
 </div><!-- end maintenance tab panel -->