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>

Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.13-slim
+
+ENV PYTHONUNBUFFERED=1
+
+# ffmpeg does the encoding; the application itself has no Python dependencies.
+RUN apt-get update \
+    && apt-get install --no-install-recommends -y ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY pyproject.toml README.md ./
+COPY music_mirror.py ./
+RUN pip install --no-cache-dir .
+
+# Runs as root by default so a bind-mounted dataset of any ownership is
+# writable. Override with `user:` in compose to run as the dataset's owner.
+ENTRYPOINT ["music-mirror"]