feat(stream): real-time transcoding for non-browser-decodable codecs

Source files in HEVC, AV1, AC3, DTS, EAC3, etc. now transcode through ffmpeg
to fragmented MP4 (h264 + aac) on-the-fly when the browser would otherwise
play silent black. Decision matrix lives in engine.DecideAction:
passthrough → remux → audio-transcode → full video-transcode.

Architecture — temp file + growing-size source:
- engine.streamSource interface abstracts byte source. Two impls:
  * diskFileSource: passthrough when codecs are already browser-friendly.
  * transcodeSource: spawns ffmpeg writing to a /tmp/tc-stream-*.mp4 file.
    A ticker polls file size and wakes blocked ReadAt callers as ffmpeg
    produces output. Estimate of final size (bitrate × duration) is
    announced over the wire so the browser's scrubber has something to
    anchor on.
- dataChannelPump now reads from streamSource instead of *os.File. HELLO
  carries Transcoding=true + an estimated total size; Seekable=true (we
  read random-access from the temp file even while writing).
- Transcoder runtime resolved per session by buildTranscodeRuntime in
  cmd/daemon: ffmpeg/ffprobe path lookup + HWAccel auto-detection
  (NVENC/QSV/VAAPI/VideoToolbox).
- New [downloads.transcode] TOML section: enabled (default true), hw_accel
  (auto), preset (veryfast), video_bitrate (5M), audio_bitrate (192k),
  max_height (optional downscale), max_concurrent (safety cap).

Falls back to passthrough if ffprobe is missing, fails, or codecs are
already browser-friendly. tmp file is cleaned up on session shutdown.

internal/cmd/daemon.go

@@ -451,6 +451,7 @@ func runDaemonStart() error {
 				webrtcRegistry.remove(sess.SessionID)
 				sessCancel()
 			}()
+			tcRuntime := buildTranscodeRuntime(ctx, cfg)
 			runCfg := engine.WebRTCStreamConfig{
 				SessionID:  sess.SessionID,
 				FilePath:   filePath,
@@ -459,6 +460,7 @@ func runDaemonStart() error {
 				ICEServers: engine.BuildICEServers(cfg.Download.WebRTC),
 				Signal:     agentClient,
 				Logger:     stdLogger{},
+				Transcode:  tcRuntime,
 			}
 			log.Printf("[wrtc %s] starting session: %s", agent.ShortID(sess.SessionID), filepath.Base(filePath))
 			if err := engine.RunWebRTCStream(sessCtx, runCfg); err != nil {