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.

app/templates/admin/admin.html

@@ -129,15 +129,18 @@
     </div>
     {% endif %}
 
-    <!-- Quick Actions Card -->
-    <div class="card">
-        <h2>⚡ Quick Actions</h2>
-        <div class="quick-actions">
-            <a href="{{ url_for('players.list') }}" class="btn btn-secondary">🖥️ View Players</a>
-            <a href="{{ url_for('content.content_list') }}" class="btn btn-secondary">📋 View Playlists</a>
-            <a href="{{ url_for('content.content_list') }}" class="btn btn-secondary">📁 View Media Library</a>
+    <!-- Maintenance Card (Admin Only) -->
+    {% if current_user.is_admin %}
+    <div class="card management-card" style="background: linear-gradient(135deg, #f7971e 0%, #ffd200 100%);">
+        <h2>🛠️ Maintenance</h2>
+        <p>Database integrity, leftover records and player authentication tokens</p>
+        <div class="card-actions">
+            <a href="{{ url_for('admin.maintenance') }}" class="btn btn-primary">
+                Open Maintenance
+            </a>
         </div>
     </div>
+    {% endif %}
 </div>
 
 <style>
@@ -228,13 +231,6 @@ body.dark-mode .stat-value {
     margin-top: 20px;
 }
 
-.quick-actions {
-    display: flex;
-    flex-direction: column;
-    gap: 10px;
-    margin-top: 15px;
-}
-
 .btn {
     padding: 10px 20px;
     text-decoration: none;