fix(stream): /critico review fixes for the sidecar cache

- ExtractSubtitlesVTTMulti: distrust output when ffmpeg is killed by signal
  (45-min timeout on a too-big remux) — a truncated WebVTT passed the len>0
  check and got cached as a silently-incomplete track until the media mtime
  changed. Skip all output on signal-kill; keep it on a clean non-zero exit.
- stream handlers: read the sidecar cache BEFORE the ffmpegPath guard so a
  pre-warmed sub/thumbnail still serves if ffmpeg was removed after the cache
  was filled.
- scan: log when the prewarm is skipped because ffmpeg is unavailable (matches
  the daemon; CLAUDE.md wants bootstrap to log on every branch).
- unexport sidecarDir/subtitleCachePath/thumbnailCachePath (no external callers).
- prewarm: surface a sample error in the summary so a systemic ffmpeg failure
  is distinguishable from one corrupt file.
- add unit tests: codec whitelist, cache paths, mtime freshness, atomic write,
  thumb-position dedup.

internal/cmd/scan.go

@@ -153,6 +153,8 @@ func runScan(dirPath string, workers int, ffprobePath string, noSync bool) error
 				CacheThumbnails: cfg.Library.CacheThumbnails,
 				Workers:         2,
 			})
+		} else {
+			fmt.Fprintf(os.Stderr, "  Skipping sidecar prewarm: ffmpeg unavailable: %v\n", err)
 		}
 	}