fix: address code review findings

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.

internal/musicbrainz/model.go

@@ -1,7 +1,5 @@
 package musicbrainz
 
-import "time"
-
 // ReleaseGroup represents a MusicBrainz Release Group entity.
 // This is the primary data model for the provider - we work with
 // Release Groups to minimize duplicates from different releases.
@@ -15,32 +13,9 @@ type ReleaseGroup struct {
 	ReleaseDate string
 }
 
-// Artist represents a MusicBrainz artist entity.
-type Artist struct {
-	ID   string
-	Name string
-}
-
-// ExternalRelease is the normalized form stored in the database,
-// matching the external_releases table schema.
-type ExternalRelease struct {
-	RGID        string
-	ArtistID    string
-	Title       string
-	Type        string
-	ReleaseDate string
-	CachedAt    time.Time
-}
-
 // ParsedReleaseGroups holds the result of parsing a MusicBrainz
 // release-group list XML response.
 type ParsedReleaseGroups struct {
 	ReleaseGroups []ReleaseGroup
 	Count         int
 }
-
-// ParsedArtist holds the result of parsing a MusicBrainz artist lookup.
-type ParsedArtist struct {
-	ID   string
-	Name string
-}