Fifth lift in the PR5 discovery-workers series. Pulls the 278-line
public-Spotify-link 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.
What the Spotify Public discovery worker does:
1. Pause enrichment workers (release shared resources).
2. For each track:
- Cancellation gate (state['cancelled']).
- Normalize artists to plain string list (handles dict + str inputs).
- Discovery cache lookup; cache hit short-circuits the search and
populates display fields from the cached match.
- 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 → match_data with 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.
4. On error: state['phase']='error' + status with error string.
5. Finally: resume enrichment workers.
This worker is structurally close to the Deezer worker (see PR5d) but
intentionally diverges on:
- Track-data field names (`spotify_public_track` vs `deezer_track`).
- Artist normalization (Spotify Public can pass dicts or strings).
- No mirrored-playlist DB writeback (sync is handled separately).
Dependencies injected via `SpotifyPublicDiscoveryDeps` (12 fields) —
spotify_public_discovery_states, spotify_client, plus 10 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).
Diff vs original after `deps.X` → global X normalization is **zero
differences** — 278 lines orig = 278 lines lifted, byte-identical body
(including all whitespace, comments, log strings).
Tests: 10 new under tests/discovery/test_discovery_spotify_public.py
covering cache hit short-circuit, dict-artist normalization, Spotify
tuple match (track/disc preservation), iTunes dict match path, Wing It
fallback, cancellation, completion phase update, activity feed entry,
top-level error handler, per-track error handling.
Full suite: 1118 passing (was 1108). Ruff clean.
c5e06691e3