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.
