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/cache.go

@@ -7,47 +7,13 @@ import (
 	"naviwatcher/internal/database"
 )
 
-// CacheStats holds the result of a cache lookup for a given artist.
-type CacheStats struct {
-	// CachedRGIDs is the list of RGIDs that are currently cached (within TTL).
-	CachedRGIDs []string
-	// CacheHitCount is the number of entries found in cache.
-	CacheHitCount int
-}
-
-// IsCached returns true if the given RGID is in the cached set.
-func (cs *CacheStats) IsCached(rgid string) bool {
-	for _, id := range cs.CachedRGIDs {
-		if id == rgid {
-			return true
-		}
-	}
-	return false
-}
-
 // GetCachedReleases queries the external_releases table for entries
 // belonging to the given artist that were cached within the specified TTL.
-// It returns a CacheStats with the list of valid RGIDs already in cache.
-func GetCachedReleases(db *database.DB, artistID string, ttl time.Duration) (*CacheStats, error) {
+// It returns the count of cached entries and any error encountered.
+func GetCachedReleases(db *database.DB, artistID string, ttl time.Duration) (int, error) {
 	releases, err := database.GetExternalReleasesByArtistWithCache(db, artistID, ttl)
 	if err != nil {
-		return nil, fmt.Errorf("get cached releases: %w", err)
+		return 0, fmt.Errorf("get cached releases: %w", err)
 	}
-
-	stats := &CacheStats{}
-	for _, r := range releases {
-		stats.CachedRGIDs = append(stats.CachedRGIDs, r.RGID)
-		stats.CacheHitCount++
-	}
-	return stats, nil
-}
-
-// IsArtistCacheValid checks whether the cache for an artist is still valid.
-// Returns true if any entries exist within the TTL for this artist.
-func IsArtistCacheValid(db *database.DB, artistID string, ttl time.Duration) (bool, error) {
-	stats, err := GetCachedReleases(db, artistID, ttl)
-	if err != nil {
-		return false, err
-	}
-	return stats.CacheHitCount > 0, nil
+	return len(releases), nil
 }