feat: Integrate WebSocket Gateway into API-based task launch

- Gateway now starts automatically when calling POST /runtime/start
- No need to run python backend/main.py separately
- Single service architecture: only FastAPI (port 8000) needed
- Gateway runs in background task and stops with pipeline
- Add error handling and logging for Gateway lifecycle

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/gateway.py

@@ -238,30 +238,45 @@ class Gateway:
             self.connected_clients.discard(websocket)
 
     async def _send_initial_state(self, websocket: ServerConnection):
-        state_payload = self.state_sync.get_initial_state_payload(
-            include_dashboard=True,
-        )
-        state_payload["data_sources"] = (
-            self._provider_router.get_usage_snapshot()
-        )
-        # Include market status in initial state
-        state_payload[
-            "market_status"
-        ] = self.market_service.get_market_status()
+        try:
+            logger.info("[Gateway] Sending initial state to client...")
+            state_payload = self.state_sync.get_initial_state_payload(
+                include_dashboard=True,
+            )
+            state_payload["data_sources"] = (
+                self._provider_router.get_usage_snapshot()
+            )
+            # Include market status in initial state
+            state_payload[
+                "market_status"
+            ] = self.market_service.get_market_status()
 
-        # Include live returns if session is active
-        if self.storage.is_live_session_active:
-            live_returns = self.storage.get_live_returns()
-            if "portfolio" in state_payload:
-                state_payload["portfolio"].update(live_returns)
+            # Include live returns if session is active
+            if self.storage.is_live_session_active:
+                live_returns = self.storage.get_live_returns()
+                if "portfolio" in state_payload:
+                    state_payload["portfolio"].update(live_returns)
 
-        await websocket.send(
-            json.dumps(
-                {"type": "initial_state", "state": state_payload},
-                ensure_ascii=False,
-                default=str,
-            ),
-        )
+            await websocket.send(
+                json.dumps(
+                    {"type": "initial_state", "state": state_payload},
+                    ensure_ascii=False,
+                    default=str,
+                ),
+            )
+            logger.info("[Gateway] Initial state sent successfully")
+        except Exception as e:
+            logger.exception(f"[Gateway] Failed to send initial state: {e}")
+            # Send error response so client knows something went wrong
+            try:
+                await websocket.send(
+                    json.dumps(
+                        {"type": "error", "message": "Failed to load initial state"},
+                        ensure_ascii=False,
+                    ),
+                )
+            except Exception:
+                pass
 
     async def _handle_client_messages(
         self,