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.

cmd/naviwatcher/main.go

@@ -45,7 +45,8 @@ func main() {
 	go func() {
 		sig := <-sigCh
 		log.Printf("Received signal %v, shutting down...", sig)
-		cancel()}()
+		cancel()
+	}()
 
 	app, err := NewApp(ctx, cfg)
 	if err != nil {
@@ -82,9 +83,6 @@ func NewApp(ctx context.Context, cfg *config.Config) (*App, error) {
 
 // Close cleans up all application resources in reverse order of initialization.
 func (a *App) Close() {
-	if a.mbClient != nil {
-		a.mbClient.Close()
-	}
 	if a.db != nil {
 		if err := a.db.Close(); err != nil {
 			log.Printf("Error closing database: %v", err)