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/DeviceHub/DeviceHubEndpoint.cs

@@ -177,6 +177,10 @@ public static class DeviceHubEndpoint
         }, ct);
     }
 
+    // Bump on every backend change so we can verify a Coolify deploy landed
+    // by looking at the hello_ack config.server_version field in the Pi journal.
+    public const string ServerVersion = "plan3-debug-2026-06-12-09";
+
     private static async Task SendHelloAckAsync(ActiveDevice active, Device device, CancellationToken ct)
     {
         var enabled = device.Config?.EnabledToolsJson ?? "[]";
@@ -186,7 +190,8 @@ public static class DeviceHubEndpoint
           "model": "{{Escape(device.Config?.Model)}}",
           "system_prompt": "{{Escape(device.Config?.SystemPrompt)}}",
           "idle_timeout_seconds": {{device.Config?.IdleTimeoutSeconds ?? 30}},
-          "enabled_tools": {{enabled}}
+          "enabled_tools": {{enabled}},
+          "server_version": "{{ServerVersion}}"
         }
         """)!.AsObject();