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/DeviceHub/DeviceHubEndpoint.cs

@@ -130,8 +130,14 @@ public static class DeviceHubEndpoint
     {
         var enabledTools = JsonSerializer.Deserialize<string[]>(device.Config?.EnabledToolsJson ?? "[]")
             ?? Array.Empty<string>();
+        // Promote any deprecated Beta-era model name to the GA name. Plan 4's
+        // admin UI will let users set a specific value; until then, anything
+        // that mentions the old preview slug routes to gpt-realtime.
+        var model = device.Config?.Model ?? "gpt-realtime";
+        if (model.StartsWith("gpt-4o-realtime-preview", StringComparison.Ordinal))
+            model = "gpt-realtime";
         var cfg = new RealtimeSettings(
-            Model: device.Config?.Model ?? "gpt-4o-realtime-preview",
+            Model: model,
             Voice: device.Config?.Voice ?? "alloy",
             SystemPrompt: device.Config?.SystemPrompt ?? "",
             IdleTimeoutSeconds: device.Config?.IdleTimeoutSeconds ?? 30,