feat(realtime): add ping/pong heartbeat and watchdog reconnect

- support ping incoming event and pong outgoing response
- add web heartbeat interval to keep ws alive
- add stale-connection watchdog to force reconnect on missing pong

app/realtime/schemas.py

@@ -17,6 +17,7 @@ RealtimeEventName = Literal[
     "message_delivered",
     "user_online",
     "user_offline",
+    "pong",
     "error",
 ]
 
@@ -40,7 +41,7 @@ class MessageStatusPayload(BaseModel):
 
 
 class IncomingRealtimeEvent(BaseModel):
-    event: Literal["send_message", "typing_start", "typing_stop", "message_read", "message_delivered"]
+    event: Literal["send_message", "typing_start", "typing_stop", "message_read", "message_delivered", "ping"]
     payload: dict[str, Any]