Add user ping system and room deletion functionality

Backend changes:
- Fix track deletion foreign key constraint (tracks.py)
  * Clear current_track_id from rooms before deleting track
  * Prevent deletion errors when track is currently playing

- Implement user ping/keepalive system (sync.py, websocket.py, ping_task.py, main.py)
  * Track last pong timestamp for each user
  * Background task sends ping every 30s, disconnects users after 60s timeout
  * Auto-pause playback when room becomes empty
  * Remove disconnected users from room_participants

- Enhance room deletion (rooms.py)
  * Broadcast room_deleted event to all connected users
  * Close all WebSocket connections before deletion
  * Cascade delete participants, queue, and messages

Frontend changes:
- Add ping/pong WebSocket handling (activeRoom.js)
  * Auto-respond to server pings
  * Handle room_deleted event with redirect to home

- Add room deletion UI (RoomView.vue, HomeView.vue, RoomCard.vue)
  * Delete button visible only to room owner
  * Confirmation dialog with warning
  * Delete button on room cards (shows on hover)
  * Redirect to home page after deletion

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/routers/tracks.py

@@ -3,12 +3,13 @@ from urllib.parse import quote
 from fastapi import APIRouter, Depends, HTTPException, status, UploadFile, File, Form, Request
 from fastapi.responses import StreamingResponse
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, func
+from sqlalchemy import select, func, update
 from mutagen.mp3 import MP3
 from io import BytesIO
 from ..database import get_db
 from ..models.user import User
 from ..models.track import Track
+from ..models.room import Room
 from ..schemas.track import TrackResponse, TrackWithUrl
 from ..services.auth import get_current_user
 from ..services.s3 import upload_file, delete_file, generate_presigned_url, can_upload_file, get_file_size, stream_file_chunks
@@ -184,10 +185,17 @@ async def delete_track(
     if track.uploaded_by != current_user.id:
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Not track owner")
 
+    # Clear current_track_id from any rooms using this track
+    await db.execute(
+        update(Room)
+        .where(Room.current_track_id == track_id)
+        .values(current_track_id=None, is_playing=False)
+    )
+
     # Delete from S3
     await delete_file(track.s3_key)
 
-    # Delete from DB
+    # Delete from DB (queue entries will be cascade deleted automatically)
     await db.delete(track)
 
     return {"status": "deleted"}