Trim GA session.update + add deploy marker + surface upstream silence

The previous run showed session_started, then total silence from the
relay for the entire conversation — no events forwarded, no errors,
nothing. Two changes to disambiguate:

1. Drop the explicit audio.input.format / audio.output.format objects.
   24 kHz PCM16 mono is the documented GA default for both directions
   and the only thing OpenAI ever advertised; an unrecognised value
   inside the format object is the most likely reason OpenAI silently
   ignores incoming audio.

2. Forward 'upstream silent NxN250ms' debug envelopes every ~2 s when
   ReceiveJsonAsync keeps returning null, and add a 'server_version'
   field to hello_ack so the Pi journal proves which build is running.

backend/Realtime/RealtimeEvents.cs

@@ -26,6 +26,11 @@ public static class RealtimeEvents
             });
         }
 
+        // Minimal GA session.update. We drop explicit audio.input/output.format
+        // objects (the GA shape accepts them but OpenAI defaults to 24 kHz PCM16
+        // mono on both directions, which is what we use). Removing the explicit
+        // format objects matches the documented minimal session config and
+        // eliminates a known source of OpenAI silently ignoring the audio.
         return new JsonObject
         {
             ["type"] = "session.update",
@@ -38,11 +43,6 @@ public static class RealtimeEvents
                 {
                     ["input"] = new JsonObject
                     {
-                        ["format"] = new JsonObject
-                        {
-                            ["type"] = "audio/pcm",
-                            ["rate"] = 24000,
-                        },
                         ["transcription"] = new JsonObject { ["model"] = "whisper-1" },
                         ["turn_detection"] = new JsonObject
                         {
@@ -50,9 +50,6 @@ public static class RealtimeEvents
                             ["threshold"] = 0.5,
                             ["prefix_padding_ms"] = 300,
                             ["silence_duration_ms"] = 500,
-                            // GA requires both explicitly — without them the
-                            // model never auto-generates a reply after VAD
-                            // detects speech_stopped.
                             ["create_response"] = true,
                             ["interrupt_response"] = true,
                         },
@@ -60,11 +57,6 @@ public static class RealtimeEvents
                     ["output"] = new JsonObject
                     {
                         ["voice"] = cfg.Voice,
-                        ["format"] = new JsonObject
-                        {
-                            ["type"] = "audio/pcm",
-                            ["rate"] = 24000,
-                        },
                     },
                 },
                 ["tools"] = tools,