feat(gateway): split video recording into silence-based segments like voice

Video (camera + screen share) DAVE stream-watch now produces per-burst
MP4 segments instead of one long .h264 per watch:
- Detects VIDEO_SILENCE_MS (4000ms) of no H264 packets → closes the
  current segment, muxes to MP4, registers in voice_recordings + uploads
  to TeleUploader, then reopens for the next burst (mirrors voice AfterSilence).
- Per-watch segment counter + per-segment depacketizer reset + closing
  guard + write-error swallow so races (silence close vs in-flight UDP
  packet) never corrupt files or crash the gateway.
- Frontend: recordings deck renders a native <video> player for MP4 rows
  (detected by filename), keeps single-playback registry across audio+video.

services/frontend/src/app/(dashboard)/recordings/view.tsx

@@ -55,6 +55,7 @@ import type {
   SpeakerSummary,
   VoiceRecording,
 } from "@/lib/types";
+import { isVideoRecording } from "@/lib/types/recording";
 import { useWebSocket } from "@/lib/ws/context";
 
 const ALL = "__all__";
@@ -557,7 +558,10 @@ export function RecordingsView({
                               {r.download_url ? (
                                 <RecordingAudioPlayer
                                   src={r.download_url}
-                                  label={`Voice recording by ${r.username}`}
+                                  label={`${
+                                    isVideoRecording(r) ? "Video" : "Voice"
+                                  } recording by ${r.username}`}
+                                  video={isVideoRecording(r)}
                                   onPlayStateChange={(active) =>
                                     setPlayingId((prev) => {
                                       if (active) return r.id;