Add track filtering, WS keepalive, and improve error handling

- Add track filtering by uploader (my tracks / all tracks) with UI tabs
- Add WebSocket ping/pong keepalive (30s interval) to prevent disconnects
- Add auto-reconnect on WebSocket close (3s delay)
- Add request logging middleware with DATABASE_URL output on startup
- Handle missing S3 files gracefully (return 404 instead of 500)
- Add debug logging for audio ended event

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

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

backend/app/services/s3.py

@@ -77,11 +77,16 @@ def get_file_content(s3_key: str) -> bytes:
     return response["Body"].read()
 
 
-def get_file_size(s3_key: str) -> int:
-    """Get file size from S3 without downloading"""
+def get_file_size(s3_key: str) -> int | None:
+    """Get file size from S3 without downloading. Returns None if file not found."""
     client = get_s3_client()
-    response = client.head_object(Bucket=settings.s3_bucket_name, Key=s3_key)
-    return response["ContentLength"]
+    try:
+        response = client.head_object(Bucket=settings.s3_bucket_name, Key=s3_key)
+        return response["ContentLength"]
+    except client.exceptions.ClientError as e:
+        if e.response['Error']['Code'] == '404':
+            return None
+        raise
 
 
 def get_file_range(s3_key: str, start: int, end: int):