Reduce heartbeat interval 25s→10s to beat upstream proxy idle timeout

Upstream SSL terminator closes idle WS connections at ~26s. Heartbeat
at T+25 was racing with the close. 10s interval keeps the connection
alive through any proxy with up to ~20s idle timeout.

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

backend/app/main.py

@@ -219,7 +219,7 @@ async def websocket_analyze(websocket: WebSocket):
                 async def _heartbeat(ws: WebSocket) -> None:
                     try:
                         while True:
-                            await asyncio.sleep(25)
+                            await asyncio.sleep(10)
                             await ws.send_json({"type": "heartbeat"})
                             logger.debug(f"[MAIN] Heartbeat sent to client {client_id}")
                     except asyncio.CancelledError: