Creates internal/musicbrainz/sync.go with SyncArtistDiscography function
that orchestrates the full flow: cache check via GetCachedReleases, fetch
from MusicBrainz API on cache miss, filter via FilterReleaseGroups, and
upsert into external_releases via database.SaveExternalRelease. Includes
SyncArtistDiscographyWithFilter variant for per-artist type filtering. All
functions support context cancellation. 14 new tests cover cache hit/miss,
status/type filtering, context cancellation, idempotency, API errors, and
full XML pipeline integration.
