perf: size the encoder pool to the CPUs the container may use

libmp3lame is single-threaded, so concurrency is one ffmpeg process per file
and the pool width is the whole of it. The width came from os.cpu_count(),
which reports the host's cores and ignores a container's cpus: allowance -- on
a 12-thread host limited to 4 CPUs that is threefold oversubscription, which
costs context switches and NAS responsiveness for no throughput.

The default now reads the cgroup v2 quota, falling back to process CPU
affinity and then to the host count. Each pass logs the number it chose.

Also stops probing every file for embedded cover art. The probe only changes
the command when a cover file sits beside the track, so ask only then; with no
cover file, -map 0:v:0? already carries embedded art if there is any. Worth
roughly 30 ms per track against about 4 s of encoding, so this is tidiness
rather than a speed-up. The per-directory cover lookup is cached alongside.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -78,6 +78,20 @@ music-mirror --source /music --mirror /music-mp3 --subdir "Artist/Album"
 `--subdir` never prunes: a partial pass cannot tell an orphan from a file
 outside its own scope.
 
+### Concurrency
+
+LAME is single-threaded — ffmpeg reports `Threading capabilities: none` for
+`libmp3lame` — so throughput comes entirely from running several encoders at
+once, one process per file. `--jobs` defaults to the CPUs the process may
+actually use, which inside a container means the `cpus:` allowance rather than
+the host's core count. Each pass logs the number it settled on.
+
+As a rough guide, a Zen 3 core encodes about 40–60× realtime at V0 depending on
+clock, so six cores clear roughly 250 hours of audio per hour of wall clock.
+The first full pass is the expensive one; after that only new and changed files
+are touched. Lower `MUSIC_MIRROR_JOBS` if you would rather the NAS stayed
+responsive than finished sooner.
+
 Requires `ffmpeg` and `ffprobe` on `PATH`. The container image provides both.
 
 ## Running it on TrueNAS Scale