feat(media): play the clip's soundtrack and share one decode pipe

A video file was picture-only: the decode pipe passed `-an`, so a starting-soon
or BRB clip with music played silent everywhere. Adds sound to every clip and
collapses the two decoders into one.

- `Services/VideoClipPipe.cs`: the shared pipe behind `MediaVideoSource` (TASK 21)
  and `AlertClipDecoder` (TASK 47) — video (`rawvideo bgra`, scaled) + audio
  (`f32le -ac 2 -ar 48000`, 50 ms chunks, byte-carry across reads), both paced to
  real time, loop restarts both so picture and sound stay together. Alert
  behaviour is unchanged.
- `MediaVideoSourceManager` applies the layer's Level/Mute per session and raises
  `AudioAvailable`; `MainViewModel` feeds it to the new
  `AudioMixer.EnqueueDesktopMediaAudio`, which joins the DESKTOP/GAME ring so the
  desktop fader and the mic auto-ducker apply exactly as they do for game audio
  and TRAX (creator ruling 2026-11-05: audio joins the group).
- Per-layer `MediaMuted` + existing `MediaVolume` surfaced as Mute/Level in the
  properties panel; `MediaMuted` persisted (ALTER TABLE guard + SELECT/INSERT).
- Level/mute are per SESSION because one decoder is shared per file path — the
  last layer touched wins; one decoder per layer would play the file twice.

Test: `MediaSourceAudioTests` — the pipe emits 50 ms chunks at 48k stereo, the
manager scales by level and drops when muted, and a media layer's sound moves the
desktop meter. Alert/decode suites unchanged and green.

Docs: task-21 closed on item 10, the "silent soundtrack" facts corrected in
frame-pipeline/output/overlays, HANDOFF rewritten. verify.sh green (529 passed;
the two documented pre-existing failures only).

Services/Audio/AudioMixer.cs

@@ -283,6 +283,25 @@ public sealed class AudioMixer : IDisposable
         LoopbackLevelChanged?.Invoke(level);
     }
 
+    /// <summary>Drops a MEDIA clip's audio into the desktop/game ring (TASK 21's audio slice).
+    /// It joins that group on purpose: the desktop fader and the mic auto-ducker then apply
+    /// exactly as they do to game audio and TRAX, so a BRB bed ducks under the creator's
+    /// voice without a second audio knob. The layer's own level/mute are applied upstream
+    /// by <see cref="MediaVideoSourceManager"/>.</summary>
+    public void EnqueueDesktopMediaAudio(AudioSample sample)
+    {
+        var data = sample.Samples;
+        if (data.Length == 0) return;
+        if (sample.SampleRate != OutputSampleRate)
+            data = ResampleLoopback(data, sample.SampleRate);
+        if (sample.Channels == 1)
+            data = UpmixToStereo(data);
+        _loopbackBuffer.Write(data);
+
+        var level = _loopbackMeter.Push(new AudioSample(data, OutputSampleRate, sample.Channels));
+        LoopbackLevelChanged?.Invoke(level);
+    }
+
     /// <summary>Drops the alert clip's audio into its own ring (TASK 47 — the
     /// stream-alert video's soundtrack, pre-scaled by the alert layer to
     /// volume × fade). Mixed at unity into the live output — NO auto-duck: an