Third lift in the PR5 discovery-workers series. Pulls the 323-line
mirrored-playlist discovery worker out of `web_server.py` into its own
focused module under `core/discovery/`. Pure 1:1 lift — wrapper keeps
the original entry-point name so the existing call site
(`_run_playlist_discovery_worker(pls, automation_id=None)` from the
automation engine) continues to work without changes.
What the playlist discovery worker does:
1. Pause enrichment workers (release shared resources).
2. Pre-compute total track count across all playlists for the automation
progress card.
3. For each playlist:
- Fast pre-scan separates already-discovered tracks (skipped, unless
incomplete metadata or a Wing It stub) from undiscovered ones.
- For each undiscovered track:
- Cancellation gate via _playlist_discovery_cancelled set.
- Discovery cache lookup (with artist validation).
- matching_engine search-query generation, then Spotify (preferred)
or iTunes (fallback) search + scoring.
- Extended search fallback (limit=50) if no high-confidence match.
- On match → enrich album from metadata cache (id, images,
total_tracks, album_type, release_date, artists, plus track_number
and disc_number), build matched_data, write to track.extra_data,
save to discovery cache.
- On miss → Wing It stub stored as 'wing_it_fallback' provider.
4. After all playlists: emit `discovery_completed` event when at least
one new track was discovered, mark automation progress 'finished'.
5. On error → automation progress 'error', traceback printed.
6. Finally: resume enrichment workers.
Dependencies injected via `PlaylistDiscoveryDeps` (16 fields) —
spotify_client, matching_engine, automation_engine, the cancellation
set, plus 12 callable helpers (pause/resume enrichment,
get_active_discovery_source, get_metadata_fallback_client/source,
update_automation_progress, get_database, get_discovery_cache_key,
validate_discovery_cache_artist, discovery_score_candidates,
get_metadata_cache, build_discovery_wing_it_stub).
Diff vs original after `deps.X` → global X normalization is **zero
differences** — 323 lines orig = 323 lines lifted, byte-identical body
(including all whitespace, comments, log strings).
Tests: 15 new under tests/discovery/test_discovery_playlist.py covering
empty playlists, no-tracks playlist skip, complete-discovery skip,
incomplete-discovery re-run, Wing It always re-run, unmatched_by_user
respect, cache hit short-circuit, match above threshold (extra_data +
cache save), match below threshold falls to Wing It, iTunes fallback,
neither-provider error path, cancellation, discovery_completed event
emit, no-event on zero-discovered, multi-playlist grand_total
aggregation.
Full suite: 1098 passing (was 1083). Ruff clean.
bda0500226