Pulls the 258-line retag worker out of `web_server.py` into a new
`core/library/` package. Pure 1:1 lift — wrapper keeps the original
entry-point name so the retag-trigger endpoint continues to work
without changes.
What `execute_retag` does:
1. Fetch album + track metadata for the new `album_id` (Spotify or
iTunes — the Spotify client transparently falls back).
2. Load existing files in the retag group from the DB.
3. Match each existing track to a new Spotify track:
- Priority 1: same disc + track number.
- Priority 2: title similarity >= 0.6 (SequenceMatcher).
4. For each matched pair:
- Re-write metadata tags via `_enhance_file_metadata`.
- Compute the new path via `_build_final_path_for_track` and move
the audio file (plus .lrc / .txt sidecars) if the path changes.
- Drop an orphaned cover.jpg if it's left in an empty directory.
- Clean up empty parent directories left behind.
- Download the new cover art into the new album dir.
5. Update the retag group record with new artist / album / image /
total_tracks / release_date and the appropriate Spotify-or-iTunes
album ID (numeric → iTunes, alphanumeric → Spotify).
6. Mark the retag state 'finished' (or 'error' on exception).
Strict 1:1 byte parity:
The original mutated `retag_state` as a module global (the function
declared `global retag_state` even though it only mutates in place).
Here `retag_state` is exposed through the `RetagDeps` proxy as a Python
property so the lifted body keeps `name[key] = value` /
`name.update(...)` syntax. The property setter rebinds the
web_server.py reference if the function ever reassigns it (currently
it doesn't, but the setter is wired for parity with the watchlist lift).
Diff vs original after `deps.X` → global X normalization is **zero
differences** apart from the dropped `global retag_state` decl and the
inline `from database.music_database import get_database` (replaced by
deps.get_database()). 258 lines orig = 258 lines lifted, byte-identical
body otherwise.
Dependencies injected via `RetagDeps` (13 fields) — config_manager,
retag_lock, spotify_client, plus 8 callable helpers
(get_audio_quality_string, enhance_file_metadata,
build_final_path_for_track, safe_move_file, cleanup_empty_directories,
download_cover_art, docker_resolve_path, get_database) and 2 property
delegates (_get_retag_state / _set_retag_state).
Tests: 11 new under tests/library/test_retag.py covering setup error
paths (no album data, no album tracks, no existing tracks),
track-number priority match, title-similarity fallback, no-match skip,
missing file skip, file move when path changes, group record update
(spotify vs iTunes ID branching by alphanumeric vs numeric album_id),
multi-disc total_discs computation.
Full suite: 1330 passing (was 1319). Ruff clean.
3a6597561a