Replace compute-only run() with an immediate sync+scan followed by a
ticker-driven periodic loop, add the sync.interval config field (default
6h) with defaults and validation, and add tests for the scheduling logic.
Add ResolveArtistMBID to query the MB artist search endpoint and return the
first matching artist ID, with httptest-backed tests for match, no-match,
HTTP error, and invalid-JSON paths.
Additive migration 008 adds an mbid column to artist_settings, extend the
ArtistSettings struct and persistence functions (SaveArtistSettings,
GetArtistSettings, GetAllArtistSettings, UpdateArtistSettings) to carry the
MusicBrainz ID, and add round-trip tests covering empty and set MBID values.
