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>

package.nix

@@ -0,0 +1,51 @@
+{
+  lib,
+  python3Packages,
+  makeWrapper,
+  ffmpeg,
+}:
+python3Packages.buildPythonApplication {
+  pname = "music-mirror";
+  inherit ((lib.importTOML ./pyproject.toml).project) version;
+  pyproject = true;
+
+  src = lib.fileset.toSource {
+    root = ./.;
+    fileset = lib.fileset.unions [
+      ./music_mirror.py
+      ./pyproject.toml
+      ./pytest.ini
+      ./tests
+    ];
+  };
+
+  build-system = [ python3Packages.setuptools ];
+  nativeBuildInputs = [ makeWrapper ];
+
+  # ffmpeg and ffprobe are called as subprocesses, so they belong on the
+  # wrapper's PATH rather than in the Python environment.
+  makeWrapperArgs = [
+    "--prefix"
+    "PATH"
+    ":"
+    (lib.makeBinPath [ ffmpeg ])
+  ];
+
+  nativeCheckInputs = [
+    python3Packages.pytestCheckHook
+    ffmpeg
+  ];
+
+  meta = {
+    description = "Maintain a lossy MP3 mirror of a lossless music library";
+    longDescription = ''
+      Reproduces a lossless library, path for path, as MP3 in a separate tree:
+      tags and cover art carried across, sources that are already MP3 copied
+      rather than re-encoded, and mirror files whose source has gone deleted.
+      The source library is never written to.
+    '';
+    homepage = "https://code.emmathe.dev/lyrathorpe/music-mirror";
+    mainProgram = "music-mirror";
+    platforms = lib.platforms.unix;
+  };
+}