When no media server is connected, discovery/sync patches sync_service's
matcher with a database-only implementation. sync_service calls it as
_find_track_in_media_server(track, candidate_pool=...) (a per-artist candidate
cache), but the database-only override took only (spotify_track) — so every
sync raised 'database_only_find_track() got an unexpected keyword argument
candidate_pool' and aborted.
Lift the override from a nested closure to a module-level
_database_only_find_track(spotify_track, candidate_pool=None) so it (a) accepts
the kwarg for interface parity with the real matcher and (b) is importable and
unit-testable. The DB-only path queries the library directly via
check_track_exists, so it accepts but doesn't need the candidate cache. Also
dropped the dead original_find_track local.
Tests: signature includes candidate_pool; called with candidate_pool={} returns
(None, 0.0) on no match; returns the match when the DB has it.
