audio: widen forward decode cushion 30/15->60/30s + add [BP-DIAG] back-pressure instrumentation

Byte cap (96MB) unchanged as the hard OOM bound; the wider time window only lets sparse Opus use existing memory headroom to ride out decode jitter. Diag logs pin whether the block is back-pressure or decode throughput.

DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs

@@ -1292,11 +1292,25 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
     /// </summary>
     private async Task DrainBackpressureAsync(CancellationToken cancellationToken)
     {
+        // [BP-DIAG] Time how long the read loop is actually parked here. A drain that releases in a few
+        // hundred ms is healthy throttling; a drain that holds for many seconds (paired with the JS
+        // PAUSED line) confirms back-pressure is parking production — whereas NO drain lines alongside
+        // frequent JS underrun PARK/RESUME lines means the loop never throttled and decode simply could
+        // not keep up. Logged only when it actually waited, so steady-state no-op drains add no noise.
+        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
+        var waited = false;
         while (IsPaused || await _audioInterop.IsProductionPaused(PlayerId))
         {
+            waited = true;
             cancellationToken.ThrowIfCancellationRequested();
             await Task.Delay(BackpressurePollMs, cancellationToken);
         }
+        if (waited)
+        {
+            _logger.LogInformation(
+                "[BP-DIAG] DrainBackpressure released after {Ms} ms (isPaused={IsPaused}, gen={Gen})",
+                stopwatch.ElapsedMilliseconds, IsPaused, _loadGeneration);
+        }
     }
 
     private async Task ThrottledNotifyStateChanged()