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/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();
@@ -25,18 +31,36 @@ 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"),
-                ["input_audio_format"] = "pcm16",
-                ["output_audio_format"] = "pcm16",
-                ["input_audio_transcription"] = new JsonObject { ["model"] = "whisper-1" },
-                ["turn_detection"] = new JsonObject
+                ["audio"] = new JsonObject
                 {
-                    ["type"] = "server_vad",
-                    ["threshold"] = 0.5,
-                    ["prefix_padding_ms"] = 300,
-                    ["silence_duration_ms"] = 500,
+                    ["input"] = new JsonObject
+                    {
+                        ["format"] = new JsonObject
+                        {
+                            ["type"] = "audio/pcm",
+                            ["rate"] = 24000,
+                        },
+                        ["transcription"] = new JsonObject { ["model"] = "whisper-1" },
+                        ["turn_detection"] = new JsonObject
+                        {
+                            ["type"] = "server_vad",
+                            ["threshold"] = 0.5,
+                            ["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",