Realtime: migrate to OpenAI GA shape (beta API was retired)

Upstream returned 'beta_api_shape_disabled' on every wake. Changes:
- Drop OpenAI-Beta: realtime=v1 header from the upgrade.
- session.update payload nests audio under session.audio.{input,output}
  and adds session.type='realtime'; audio.{input,output}.format is now
  an object {type:audio/pcm, rate:24000} instead of 'pcm16' string.
- Default model bumped to gpt-realtime (the GA name); the relay also
  promotes any stored 'gpt-4o-realtime-preview*' to gpt-realtime so
  pre-Plan-3 DeviceConfig rows keep working without a manual DB edit.

backend/Realtime/OpenAIRealtimeUpstream.cs

@@ -11,7 +11,8 @@ public class OpenAIRealtimeUpstream : IRealtimeUpstream
     public async Task ConnectAsync(string apiKey, string model, CancellationToken ct)
     {
         _ws.Options.SetRequestHeader("Authorization", $"Bearer {apiKey}");
-        _ws.Options.SetRequestHeader("OpenAI-Beta", "realtime=v1");
+        // The GA Realtime API does NOT take the OpenAI-Beta: realtime=v1 header.
+        // Sending it returns upstream error code=beta_api_shape_disabled.
         var uri = new Uri($"wss://api.openai.com/v1/realtime?model={Uri.EscapeDataString(model)}");
         await _ws.ConnectAsync(uri, ct);
     }