Add admin maintenance console

Admin-only page reachable from a new card on the admin panel (replacing the
Quick Actions card) that centralises housekeeping operations:

* Database integrity: PRAGMA integrity_check / quick_check / foreign_key_check,
  plus VACUUM + ANALYZE to reclaim free space.
* Leftover records: detects and removes orphaned rows left behind by deleted
  players/content (player_feedback, player_edit, playlist_content, dangling
  player.playlist_id) and orphaned edited-media folders on disk.
* Player authentication tokens: manual reset (single or all), a configurable
  automatic reset interval, and cancellation of a pending reset.

Resetting a token rotates player.auth_code and sets auth_reset_required, so the
next heartbeat (POST /api/player-feedback) answers 401 + auth_required and the
client restarts its authentication flow. A successful authentication clears the
flag and stamps auth_token_issued_at, from which the interval is measured.

New: PlayerAuthConfig model, add_player_auth_reset migration (registered in
docker-entrypoint.sh), admin/maintenance.html.

docker-entrypoint.sh

@@ -84,6 +84,7 @@ MIGRATIONS=(
     "add_url_to_content.py"
     "add_original_filename_to_content.py"
     "add_deployment_fields_to_player.py"
+    "add_player_auth_reset.py"
 )
 
 echo "Running database migrations..."