feat: mirror a lossless library to MP3 for iPod sync

Apple's Music app cannot read FLAC, so getting a lossless library onto an iPod
requires a converted copy somewhere. This keeps that copy beside the library on
a NAS rather than on a laptop, and keeps it current unattended.

Walks a source tree and reproduces it path for path as MP3: tags and cover art
carried across, already-MP3 sources copied rather than re-encoded, and mirror
files whose source has gone deleted along with any directories they emptied.
The source library is never written to.

Freshness is tracked by modification time -- an encoded file is stamped with
its source's mtime, so a file is stale exactly when the two differ. That keeps
runs idempotent without a database that could fall out of step with whatever
owns the library, which here is Lidarr.

Encodes go to a temporary file and are renamed into place, so an interrupted
run cannot leave a truncated MP3 that the next run mistakes for finished work.
A lock file in the mirror root prevents overlapping passes.

Ships as a Python package with a console script, a container image with a
TrueNAS Scale compose file, and a Nix flake providing the package, an overlay
and a dev shell. The test suite runs real ffmpeg encodes rather than mocks --
the failures worth catching are in what ffmpeg does with tags, cover art and
container formats.

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

compose.yaml

@@ -0,0 +1,31 @@
+# TrueNAS Scale "Custom App" definition.
+#
+# Build the image on the NAS first (there is no published image yet):
+#   git clone https://code.emmathe.dev/lyrathorpe/music-mirror
+#   cd music-mirror && docker build -t music-mirror:latest .
+#
+# Adjust the two host paths and the user to match your pool. The source is
+# mounted read-only on purpose: nothing here should ever be able to write to
+# the lossless library.
+services:
+  music-mirror:
+    image: music-mirror:latest
+    container_name: music-mirror
+    restart: unless-stopped
+    # The dataset owner, so the mirror is not written as root. `id apps` or the
+    # ownership of the mirror dataset will tell you the right numbers.
+    user: "568:568"
+    environment:
+      MUSIC_MIRROR_SOURCE: /music
+      MUSIC_MIRROR_MIRROR: /mirror
+      # LAME V0 averages about 245 kbps. Use 256 for constant bitrate instead.
+      MUSIC_MIRROR_QUALITY: V0
+      # How long to wait between passes. Lidarr imports are picked up on the
+      # next one.
+      MUSIC_MIRROR_INTERVAL: 6h
+      # Concurrent encodes; defaults to the CPU count. Lower it to leave the
+      # NAS responsive during the first full pass.
+      # MUSIC_MIRROR_JOBS: "4"
+    volumes:
+      - /mnt/tank/media/music:/music:ro
+      - /mnt/tank/media/music-mp3:/mirror