Add missing update-duration endpoint for playlist content

- Create new endpoint to handle duration updates for playlist content items
- Validates duration is at least 1 second
- Updates duration in playlist_content association table
- Increments playlist version for sync tracking
- Fixes issue where duration spinner buttons were returning save errors

app/blueprints/content.py

@@ -458,6 +458,56 @@ def update_playlist_content_edit_enabled(playlist_id: int, content_id: int):
         return jsonify({'success': False, 'message': str(e)}), 500
 
 
+@content_bp.route('/playlist/<int:playlist_id>/update-duration/<int:content_id>', methods=['POST'])
+@login_required
+def update_playlist_content_duration(playlist_id: int, content_id: int):
+    """Update content duration in playlist."""
+    playlist = Playlist.query.get_or_404(playlist_id)
+    
+    try:
+        content = Content.query.get_or_404(content_id)
+        
+        # Get duration from request
+        try:
+            duration = int(request.form.get('duration', 10))
+        except (ValueError, TypeError):
+            return jsonify({'success': False, 'message': 'Invalid duration value'}), 400
+        
+        # Validate duration (minimum 1 second)
+        if duration < 1:
+            return jsonify({'success': False, 'message': 'Duration must be at least 1 second'}), 400
+        
+        from app.models.playlist import playlist_content
+        from sqlalchemy import update
+        
+        # Update duration in association table
+        stmt = update(playlist_content).where(
+            (playlist_content.c.playlist_id == playlist_id) &
+            (playlist_content.c.content_id == content_id)
+        ).values(duration=duration)
+        db.session.execute(stmt)
+        
+        # Increment playlist version
+        playlist.increment_version()
+        
+        db.session.commit()
+        cache.clear()
+        
+        log_action('info', f'Updated duration={duration}s for "{content.filename}" in playlist "{playlist.name}"')
+        
+        return jsonify({
+            'success': True,
+            'message': 'Duration updated',
+            'duration': duration,
+            'version': playlist.version
+        })
+        
+    except Exception as e:
+        db.session.rollback()
+        log_action('error', f'Error updating duration: {str(e)}')
+        return jsonify({'success': False, 'message': str(e)}), 500
+
+
 @content_bp.route('/upload-media-page')
 @login_required
 def upload_media_page():