feat(trickplay): scan-time montage sprite for the web scrubber

Pre-generate ONE trickplay sprite (montage JPEG of frames sampled every
library.trickplay.interval, default 10s) + a JSON manifest per file during the
scan/auto-scan prewarm, cached in .unarr next to the media. The web scrubber
shows tiles from it instead of extracting frames live — removing the ffmpeg
contention with the active stream that broke seekbar previews (the original
'no thumbnail' report was the auto-scan prewarm decoding the same file the HLS
transcode was reading, not a seek-index fault).

- config: [library.trickplay] enabled/interval/width (default on, 10s, 240px),
  editable + a toggle; IntervalSeconds() with a 10s fallback.
- mediainfo: GenerateTrickplay (one ffmpeg fps=1/interval,scale,tile pass; idle
  I/O priority; ceil() frame count so no black trailing tile; a 16.7M-px cap
  coarsens the interval for long media so a single sprite stays decodable on
  iOS/Safari) + sprite/manifest sidecar cache helpers.
- engine: /trickplay endpoint (manifest JSON, ?kind=sprite JPEG); the agent owns
  the tile width so the web requests by path only; thumb:<sha256> token reused.
- prewarm: a trickplay job per item, gated; scan.go + daemon.go wire the config.

Tests: parseDims; synthetic 3x2 / exact-multiple / 1x1; real-file e2e smoke
(S02E08 → 143 tiles, 662KB sprite). Non-breaking: the existing 5-frame panel
prewarm + on-demand /thumbnail stay until the web migrates to the sprite.

internal/cmd/scan.go

@@ -144,14 +144,17 @@ func runScan(dirPath string, workers int, ffprobePath string, noSync bool) error
 	// ".unarr" dir so playback gets instant subtitles/thumbnails and huge remuxes
 	// never hit the on-demand timeout. Best-effort + Ctrl-C interruptible (the scan
 	// itself is already saved).
-	if cfg.Library.CacheSubtitles || cfg.Library.CacheThumbnails {
+	if cfg.Library.CacheSubtitles || cfg.Library.CacheThumbnails || cfg.Library.Trickplay.Enabled {
 		if ff, err := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath); err == nil {
 			fmt.Fprintf(os.Stderr, "  Pre-extracting subtitles + thumbnails to cache… (Ctrl-C to skip)\n")
 			library.PrewarmSidecars(ctx, cache, library.PrewarmOptions{
-				FFmpegPath:      ff,
-				CacheSubtitles:  cfg.Library.CacheSubtitles,
-				CacheThumbnails: cfg.Library.CacheThumbnails,
-				Workers:         2,
+				FFmpegPath:           ff,
+				CacheSubtitles:       cfg.Library.CacheSubtitles,
+				CacheThumbnails:      cfg.Library.CacheThumbnails,
+				Workers:              2,
+				Trickplay:            cfg.Library.Trickplay.Enabled,
+				TrickplayIntervalSec: cfg.Library.Trickplay.IntervalSeconds(),
+				TrickplayWidth:       cfg.Library.Trickplay.Width,
 			})
 		} else {
 			fmt.Fprintf(os.Stderr, "  Skipping sidecar prewarm: ffmpeg unavailable: %v\n", err)