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/lib/types/recording.ts

@@ -16,6 +16,11 @@ export interface VoiceRecording {
   transcription?: string | null;
 }
 
+/** True when the recording file is video (camera/screen-share MP4). */
+export function isVideoRecording(r: Pick<VoiceRecording, "filename">): boolean {
+  return /\.(mp4|webm|mov|h264)$/i.test(r.filename);
+}
+
 export interface PaginatedRecordings {
   items: VoiceRecording[];
   nextCursor: string | null;