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/RealtimeSession.cs

@@ -128,10 +128,18 @@ public class RealtimeSession
             if (evt is null && ct.IsCancellationRequested) return;
             if (evt is null)
             {
-                // Upstream has gone silent. If null persists for >2s, log so
-                // we can spot a dead upstream that's not surfacing as a close.
+                // Upstream has gone silent. Surface that to the device every
+                // ~2s so we can tell "OpenAI is up but emitting nothing" from
+                // "the upstream WS was closed" — Coolify logs aren't reachable.
                 if (++nullPolls % 8 == 0)
-                    _logger.LogDebug("upstream silent ({Polls} x 250ms polls)", nullPolls);
+                {
+                    _logger.LogInformation("upstream silent ({Polls} x 250ms polls)", nullPolls);
+                    await _output.WriteEnvelopeAsync(new JsonObject
+                    {
+                        ["type"] = "debug",
+                        ["message"] = "upstream silent " + nullPolls + "x250ms",
+                    }, ct);
+                }
                 continue;
             }
             nullPolls = 0;