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/celery_app.py

@@ -71,7 +71,6 @@ def make_celery(app=None):
             'app.tasks.scan.cleanup_old_tasks': {'queue': 'maintenance'},
             'app.tasks.scan.update_system_stats': {'queue': 'maintenance'},
             'app.tasks.scan.update_batch_stats': {'queue': 'maintenance'},
-            'app.tasks.maintenance.sync_character_fandoms': {'queue': 'maintenance'},
 
             # Import tasks - handled by worker (heavy processing)
             'app.tasks.import_file.*': {'queue': 'import'},