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 {

internal/cmd/webrtc_session_registry.go

@@ -4,6 +4,10 @@ import (
 	"context"
 	"log"
 	"sync"
+
+	"github.com/torrentclaw/unarr/internal/config"
+	"github.com/torrentclaw/unarr/internal/engine"
+	"github.com/torrentclaw/unarr/internal/library/mediainfo"
 )
 
 // webrtcRegistry tracks per-session cancel funcs for active custom WebRTC
@@ -60,3 +64,43 @@ type stdLogger struct{}
 func (stdLogger) Infof(format string, args ...any)  { log.Printf(format, args...) }
 func (stdLogger) Warnf(format string, args ...any)  { log.Printf("WARN: "+format, args...) }
 func (stdLogger) Errorf(format string, args ...any) { log.Printf("ERROR: "+format, args...) }
+
+// buildTranscodeRuntime resolves the ffmpeg/ffprobe binaries + config knobs
+// for the WebRTC streaming pipeline. Failure to resolve a binary returns a
+// runtime with empty paths so engine.RunWebRTCStream falls back to
+// passthrough — the user gets a clearer codec error from the browser than a
+// daemon-side abort.
+func buildTranscodeRuntime(ctx context.Context, cfg config.Config) engine.TranscodeRuntime {
+	if !cfg.Download.Transcode.Enabled {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	ffmpegPath, errF := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath)
+	ffprobePath, errP := mediainfo.ResolveFFprobe(cfg.Library.FFprobePath)
+	if errF != nil || errP != nil {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	hw := engine.HWAccelNone
+	switch cfg.Download.Transcode.HWAccel {
+	case "auto":
+		hw = engine.DetectHWAccel(ctx, ffmpegPath)
+	case "nvenc":
+		hw = engine.HWAccelNVENC
+	case "qsv":
+		hw = engine.HWAccelQSV
+	case "vaapi":
+		hw = engine.HWAccelVAAPI
+	case "videotoolbox":
+		hw = engine.HWAccelVideoToolbox
+	case "none", "":
+		hw = engine.HWAccelNone
+	}
+	return engine.TranscodeRuntime{
+		FFmpegPath:   ffmpegPath,
+		FFprobePath:  ffprobePath,
+		HWAccel:      hw,
+		Preset:       cfg.Download.Transcode.Preset,
+		VideoBitrate: cfg.Download.Transcode.VideoBitrate,
+		AudioBitrate: cfg.Download.Transcode.AudioBitrate,
+		MaxHeight:    cfg.Download.Transcode.MaxHeight,
+	}
+}