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/engine/transcoder.go

@@ -64,6 +64,26 @@ func NewTranscoder(ctx context.Context, filePath string, opts TranscodeOpts) (*T
 	return t, nil
 }
 
+// startTranscoderToFile spawns ffmpeg with a pre-built argv where the last
+// argument is an output file path (instead of pipe:1). Used by streamSource
+// when we want random-access reads against a growing temp file rather than
+// sequential pipe consumption.
+func startTranscoderToFile(ctx context.Context, ffmpegPath string, args []string, t *Transcoder) (*Transcoder, error) {
+	if ffmpegPath == "" {
+		return nil, fmt.Errorf("transcoder: empty ffmpeg path")
+	}
+	cmd := exec.CommandContext(ctx, ffmpegPath, args...)
+	if t == nil {
+		t = &Transcoder{}
+	}
+	t.cmd = cmd
+	cmd.Stderr = &errWriter{t: t}
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("transcoder: start ffmpeg: %w", err)
+	}
+	return t, nil
+}
+
 // Read implements io.Reader.
 func (t *Transcoder) Read(p []byte) (int, error) { return t.out.Read(p) }