Fourth lift in the PR5 discovery-workers series. Pulls the 270-line
Deezer 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 sites continue to work
without changes.
What the Deezer discovery worker does:
1. Pause enrichment workers (release shared resources).
2. For each Deezer track:
- Cancellation gate (state['cancelled']).
- Discovery cache lookup; cache hit short-circuits the search and
populates display fields from the cached match (artist string,
album name).
- SimpleNamespace duck-type → `_search_spotify_for_tidal_track`
(shared search helper, returns tuple for Spotify or dict for iTunes).
- On Spotify match: build `match_data` preserving track_number /
disc_number from raw API data, image extracted from album images
or track object fallback, release_date filled from track.release_date
when album dict is missing it.
- On iTunes match: dict result populated as `match_data`, source set
to discovery_source, image extracted from album images.
- Save matched result to discovery cache.
- On miss: Wing It stub stored as 'wing-it' status.
3. After all tracks: phase='discovered', activity feed entry, sync
discovery results back to mirrored playlist via
`_sync_discovery_results_to_mirrored`.
4. On error: state['phase']='error' + status with error string.
5. Finally: resume enrichment workers.
Dependencies injected via `DeezerDiscoveryDeps` (13 fields) —
deezer_discovery_states dict, spotify_client, plus 11 callable helpers
(pause/resume enrichment, get_active_discovery_source,
get_metadata_fallback_client, get_discovery_cache_key, get_database,
validate_discovery_cache_artist, search_spotify_for_tidal_track,
build_discovery_wing_it_stub, add_activity_item,
sync_discovery_results_to_mirrored).
Diff vs original after `deps.X` → global X normalization is **zero
differences** — 270 lines orig = 270 lines lifted, byte-identical body
(including all whitespace, comments, log strings).
Tests: 10 new under tests/discovery/test_discovery_deezer.py covering
cache hit short-circuit, Spotify tuple match (track/disc number
preservation), iTunes dict match path, Wing It fallback, cancellation,
completion phase update, activity feed entry, mirrored sync invocation,
top-level error handler, per-track error handling.
Full suite: 1108 passing (was 1098). Ruff clean.
