feat(streaming): seek-restart, single-session, idle sweeper, probe.json

Follow-ups on the daemon HLS pipeline (0fc0e1c):

- engine/hls.go HLSSession.Register now closes every other active
  session in the registry. Modeled as "one viewer == one transcode" so
  repeated quality switches or page reloads don't leave orphan ffmpegs
  saturating the CPU until the idle sweeper reaps them 30 min later.
- engine/hls.go restartFromSegment kills + respawns ffmpeg with
  -ss / -output_ts_offset / -start_number when the browser asks for a
  segment far ahead of the writer head. Segments already on disk stay
  cached. Without this, a user dragging the scrubber to minute 30 of a
  fresh stream blocks until the encoder reaches minute 30 in real time.
- engine/hls.go subtitle disambiguation: never set DEFAULT=YES on any
  rendition (anime forced "signs only" tracks were autoselected and
  rendered nothing during opening dialogue, looking broken). Names get
  numeric suffixes when language is duplicated; FORCED tracks get a
  "(forzados)" suffix.
- engine/hls.go ProbeInfo() exposes codec / audio / subtitle metadata
  to the new GET /hls/<id>/probe.json endpoint for the player's info
  badge + bandwidth logic.
- engine/hls.go scale chain fix: chains a trunc(iw/2)*2 scale after
  the height cap so libx264 stops rejecting odd widths (853x480 etc.).
- engine/hls.go HW encoder tuning: NVENC -preset p4 -rc vbr -tune hq,
  QSV -preset medium.
- engine/stream_server.go routes /hls/<id>/probe.json to the session.
- cmd/daemon.go runs an idle sweeper goroutine every 5 min, reaping
  sessions whose last segment fetch was >30 min ago.

internal/cmd/daemon.go

@@ -513,6 +513,25 @@ func runDaemonStart() error {
 		}
 	}()
 
+	// Periodic HLS session sweeper (every 5 min). Closes sessions whose last
+	// segment fetch was over 30 min ago — kills the orphan ffmpeg + removes
+	// the per-session tmpdir, so a tab that died mid-stream doesn't leak
+	// disk space until daemon shutdown.
+	go func() {
+		ticker := time.NewTicker(5 * time.Minute)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				if n := streamSrv.HLS().SweepIdle(); n > 0 {
+					log.Printf("[hls] swept %d idle session(s)", n)
+				}
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
 	// Start auto-scan goroutine
 	scanPaths := daemonCfg.ScanPaths
 	if len(scanPaths) > 0 && cfg.Library.AutoScan {