Implements ScanArtist/ScanAll in internal/scanner loading local/external
releases via the database layer with ctx-cancellation checks, plus a
compute-only run() hook that logs missing-release counts. Add table-driven
tests using in-memory SQLite fixtures.
Remove dead code: duplicate ExternalRelease/Artist/ParsedArtist structs in
model.go, ParseArtist/mbArtist/mbArtistData in client.go, ArtistTypeFilter
and related filtering functions in api.go, SyncArtistDiscographyWithFilter
in sync.go, and CacheStats/IsArtistCacheValid in cache.go.
Fix bugs: SaveExternalRelease now stores NULL instead of empty string for
zero CachedAt; sync upserts are now transactional with stale release cleanup;
getCachedReleases returns int instead of *CacheStats; doGet uses url.Values
for proper query encoding of MBID.
Fix tests: removed duplicate TestRun_GracefulShutdown, removed dead code
(_ = dbPath) from TestNewApp, fixed assertions in httptest handler goroutine
to avoid data race, increased rate limiter timing tolerance, removed
Client.Close() calls (no-op removed), fixed sync test cache expiry to use
UPDATE instead of 0 TTL races.
Fix formatting: cancel()}() formatting in main.go, error format string in sync.go.
