fix: address third code review findings

- Prevent infinite pagination loop when API returns empty release groups page
- Move NormalizeString regexes to package level to avoid recompilation on every call
- Use UTC consistently for cached_at timestamps to avoid DST-related TTL skew

internal/database/external_releases.go

@@ -30,7 +30,7 @@ func GetExternalRelease(db *DB, rgid string) (*ExternalRelease, error) {
 func SaveExternalRelease(db *DB, release *ExternalRelease) error {
 	var cachedAt interface{}
 	if !release.CachedAt.IsZero() {
-		cachedAt = release.CachedAt.Format("2006-01-02 15:04:05")
+		cachedAt = release.CachedAt.UTC().Format("2006-01-02 15:04:05")
 	}
 	_, err := db.Conn().Exec(
 		"INSERT OR REPLACE INTO external_releases (rgid, artist_id, title, type, release_date, is_ignored, cached_at) VALUES (?, ?, ?, ?, ?, ?, ?)",
@@ -145,7 +145,7 @@ func CountExternalReleases(db *DB) (int, error) {
 // GetExternalReleasesByArtistWithCache returns cached external_release rows for a given artist_id
 // that are within the specified TTL.
 func GetExternalReleasesByArtistWithCache(db *DB, artistID string, ttl time.Duration) ([]ExternalRelease, error) {
-	cutoff := time.Now().Add(-ttl)
+	cutoff := time.Now().UTC().Add(-ttl)
 	rows, err := db.Conn().Query(
 		"SELECT rgid, artist_id, title, type, release_date, is_ignored, cached_at FROM external_releases WHERE artist_id = ? AND cached_at >= ?",
 		artistID, cutoff.Format("2006-01-02 15:04:05"),