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/Settings/SystemSettings.cs

@@ -6,6 +6,6 @@ public class SystemSettings
     public string DefaultSystemPrompt { get; set; } =
         "You are a helpful voice assistant. Keep responses concise.";
     public string DefaultVoice { get; set; } = "alloy";
-    public string DefaultModel { get; set; } = "gpt-4o-realtime-preview";
+    public string DefaultModel { get; set; } = "gpt-realtime";
     public int DefaultIdleTimeoutSeconds { get; set; } = 30;
 }