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/main.py

@@ -612,13 +612,6 @@ def trigger_recompute_all_centroids():
     return redirect(url_for('main.settings', tab='maintenance'))
 
 
-@main.post('/settings/maintenance/sync-character-fandoms')
-def trigger_sync_character_fandoms():
-    from app.tasks.maintenance import sync_character_fandoms
-    sync_character_fandoms.apply_async(queue='maintenance')
-    return redirect(url_for('main.settings', tab='maintenance'))
-
-
 # ----------------------------
 # Tag add/remove endpoints
 # ----------------------------