- split metadata lookup logic into core/metadata/*
- keep core/metadata_service.py as the legacy barrel
- update tests and artist-detail code to patch concrete modules
JohnBaumb's review: "If we're going to refactor the web_server.py soon,
might as well start moving stuff away from web_server.py in our PRs.
_build_source_only_artist_detail, make it a module, it's perfect."
This continues the pattern the prior commit started with the source-ID
lookup helpers: move the pure data-building logic to a side-effect-free
core module, leave a thin wrapper in web_server.py that bridges the
Flask response and the module-global clients.
**core/artist_source_detail.py** — pure function that takes the artist id,
name, and source plus dependency-injected per-source clients (spotify,
deezer, itunes, discogs) and a Last.fm API key. Returns
(payload_dict, http_status) so it isn't coupled to Flask.
**web_server.py wrapper** — builds the client bag from the module globals
(checks Spotify auth, constructs the Discogs client from the configured
token, reads the Last.fm API key) and wraps the core return in jsonify.
147 lines of logic go away from web_server.py; the 24-line wrapper is
purely glue.
**tests/test_artist_source_detail.py** — 21 focused tests covering the
response envelope, the source-specific ID-field stamping for all six
supported sources, the dedup_variants=False contract (the behaviour
that originally motivated the split of MetadataLookupOptions), per-source
genre/follower extraction with safe handling of missing or throwing
clients, and the Last.fm enrichment branch including the no-key and
error-path cases. Runtime 0.26s.
