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>

pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["setuptools>=77"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "music-mirror"
+version = "0.1.0"
+description = "Maintain a lossy MP3 mirror of a lossless music library"
+readme = "README.md"
+requires-python = ">=3.11"
+# No runtime Python dependencies: the work is done by ffmpeg, which must be on
+# PATH.
+dependencies = []
+
+[project.scripts]
+music-mirror = "music_mirror:run"
+
+[project.urls]
+Homepage = "https://code.emmathe.dev/lyrathorpe/music-mirror"
+
+[tool.setuptools]
+py-modules = ["music_mirror"]