Adds ``discover_tracks(tracks) -> List[NormalizedTrack]`` to the
PlaylistSource interface. Sources whose tracks already carry
provider IDs (Spotify, Tidal, Qobuz, YouTube, Deezer, Spotify
public, iTunes link, SoulSync Discovery) inherit a no-op default;
ListenBrainz + Last.fm override to run the matching engine.
This closes the last gap before LB / Last.fm / SoulSync Discovery
can land as Sync-page mirror sources: the refresh handler now
calls ``source.discover_tracks(...)`` whenever a source returns
tracks with ``needs_discovery=True``, so mirrored LB rows arrive
already discovered + ready for the sync pipeline. Previously, LB
playlists ran through a separate state-machine worker tied to the
Discover-page UI, with results stored in ``discovery_cache``
instead of ``mirrored_playlist_tracks.extra_data``.
Changes:
- ``core/playlists/sources/base.py`` — PlaylistSource switches from
Protocol to ABC so a concrete default for ``discover_tracks``
can live on the base class. The four real-work methods stay
``@abstractmethod``; instantiating an adapter that forgets one
fails loudly at construction.
- ``core/discovery/matching.py`` (new) — pure ``match_mb_tracks``
helper that runs Strategy-1-only matching-engine queries against
Spotify (primary) or iTunes (fallback). No state machine, no
discovery-cache writes, no wing-it stub — that richer flow stays
in ``core/discovery/listenbrainz.py`` for the Discover-page UI.
- ``ListenBrainzPlaylistSource`` + ``LastFMPlaylistSource`` take
an optional ``discover_callable`` constructor arg. Last.fm reuses
the LB implementation since the track shape is identical.
- ``bootstrap.build_playlist_source_registry`` accepts a
``discover_callable`` kwarg and wires it into LB + Last.fm
adapters.
- ``web_server.py`` boot constructs the discovery callable from the
existing matching engine + ``_discovery_score_candidates`` +
Spotify / iTunes clients, passes through to the registry.
- ``refresh_mirrored.py`` adds a small ``_maybe_discover`` helper
that calls ``source.discover_tracks(...)`` between fetch and
``to_mirror_track_dict`` projection — only fires when at least
one track has ``needs_discovery=True``, so the normal Spotify /
Tidal / etc. refresh path stays a zero-cost pass-through.
Tests:
- 5 new adapter tests: default no-op pass-through, LB discovery
with mixed matches/misses, LB no-callable fallback, Last.fm
shares the LB implementation, mirror-dict spotify_hint emit.
- 1 new automation test: end-to-end LB refresh with a stub
discover_callable proves the matched_data lands in
``mirror_playlist_tracks.extra_data`` after the registry
refresh + discover hop.
225 tests across adapter + automation suites green.
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