feat(maintenance): backfill fandom tags onto legacy character attachments

New Celery task sync_character_fandoms_to_images on the maintenance
queue. For every character tag with a non-null fandom_id, runs the
same INSERT ... SELECT ... ON CONFLICT DO NOTHING that set_tag_fandom
already runs inline, attaching the fandom tag to every image that
has the character attached.

Why this exists: migration j26042101 extracted the '(Fandom)' suffix
from pre-refactor character names into tag.fandom_id, but it did NOT
walk image_tags to attach the fandom rows to every image that had
the character. Post-refactor auto-apply (in add_tag /
accept_image_suggestion / set_tag_fandom) only fires on NEW
add/accept events — pre-existing character attachments from before
the migration still show the character pill in the modal but no
fandom pill, because the fandom row was never added to image_tags.

Surfaced by Settings → Maintenance → 'Sync fandoms to images'.
Safe, additive, idempotent. Run once after the migration to close
the gap; subsequent set-fandom operations continue to maintain the
invariant inline.

Verified locally: seeded a character+fandom pair with only the
character attached to image 1; task added the fandom to image_tags;
final state has both rows.

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

app/templates/settings.html

@@ -456,6 +456,18 @@
         <button type="submit" class="btn secondary-btn">Recompute all centroids</button>
       </form>
 
+      <p class="settings-hint mt-paragraph">
+        For every character tag with a fandom assigned, attach the fandom
+        tag to every image already tagged with that character. Additive
+        only — never removes fandom attachments. Safe to re-run. Backfills
+        data migrated from pre-refactor tag names that embedded the
+        "(Fandom)" suffix, since the migration set tag.fandom_id but did
+        not retroactively attach the fandom row to image_tags.
+      </p>
+      <form action="{{ url_for('main.trigger_sync_character_fandoms_to_images') }}" method="post" onsubmit="return confirm('Attach each character\\'s fandom tag to every image with that character?');">
+        <button type="submit" class="btn secondary-btn">Sync fandoms to images</button>
+      </form>
+
     </section>
 
     <section class="settings-section">