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/musicbrainz/sync.go

@@ -55,7 +55,7 @@ func SyncArtistDiscography(
 	filtered := FilterReleaseGroups(groups)
 
 	// Step 5: Upsert within a transaction — delete old entries first, then insert new ones.
-	now := time.Now()
+	now := time.Now().UTC()
 	tx, err := db.Begin()
 	if err != nil {
 		return nil, fmt.Errorf("sync artist discography: begin transaction: %w", err)