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,

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);
     }

backend/Realtime/RealtimeEvents.cs

@@ -6,6 +6,12 @@ namespace backend.Realtime;
 
 public static class RealtimeEvents
 {
+    // GA Realtime API shape (the Beta API shape was retired in 2026 with
+    // upstream error "beta_api_shape_disabled"). Key differences:
+    //   - session.type = "realtime" is required.
+    //   - voice / audio formats / transcription / turn_detection now nested
+    //     under session.audio.{input,output}.
+    //   - No OpenAI-Beta header on the upgrade (see OpenAIRealtimeUpstream).
     public static JsonObject SessionUpdate(RealtimeSettings cfg, IEnumerable<ITool> enabledTools)
     {
         var tools = new JsonArray();
@@ -25,12 +31,19 @@ public static class RealtimeEvents
             ["type"] = "session.update",
             ["session"] = new JsonObject
             {
-                ["voice"] = cfg.Voice,
+                ["type"] = "realtime",
+                ["model"] = cfg.Model,
                 ["instructions"] = cfg.SystemPrompt,
-                ["modalities"] = new JsonArray("text", "audio"),
+                ["audio"] = new JsonObject
-                ["input_audio_format"] = "pcm16",
+                {
-                ["output_audio_format"] = "pcm16",
+                    ["input"] = new JsonObject
-                ["input_audio_transcription"] = new JsonObject { ["model"] = "whisper-1" },
+                    {
+                        ["format"] = new JsonObject
+                        {
+                            ["type"] = "audio/pcm",
+                            ["rate"] = 24000,
+                        },
+                        ["transcription"] = new JsonObject { ["model"] = "whisper-1" },
                         ["turn_detection"] = new JsonObject
                         {
                             ["type"] = "server_vad",
@@ -38,6 +51,17 @@ public static class RealtimeEvents
                             ["prefix_padding_ms"] = 300,
                             ["silence_duration_ms"] = 500,
                         },
+                    },
+                    ["output"] = new JsonObject
+                    {
+                        ["voice"] = cfg.Voice,
+                        ["format"] = new JsonObject
+                        {
+                            ["type"] = "audio/pcm",
+                            ["rate"] = 24000,
+                        },
+                    },
+                },
                 ["tools"] = tools,
                 ["tool_choice"] = "auto",
             },

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;
 }