fix: remove OpenAI-Beta header, use gpt-realtime-2 GA model

The old OpenAI-Beta: realtime=v1 header is rejected by the GA API.
Removing it via extra_headers override. Using gpt-realtime-2 which
is the current production Realtime model.

backend/services/realtime.py

@@ -58,7 +58,8 @@ class RealtimeRelay:
 
             logger.info("Connecting to OpenAI Realtime API...")
             async with client.beta.realtime.connect(
-                model="gpt-4o-mini-realtime-preview-2025-07-18",
+                model="gpt-realtime-2",
+                extra_headers={"OpenAI-Beta": ""},
             ) as conn:
                 self._conn = conn
                 self._connected = True