chore: remove sync_character_fandoms maintenance task

tag.fandom_id is now authoritative. set_tag_fandom auto-applies the
fandom tag inline; add_tag already did. There's no drift to sync on
a schedule anymore.

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

app/templates/settings.html

@@ -456,14 +456,6 @@
         <button type="submit" class="btn secondary-btn">Recompute all centroids</button>
       </form>
 
-      <p class="settings-hint mt-paragraph">
-        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>