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/api.go

@@ -26,6 +26,14 @@ var includedTypes = map[string]bool{
 	"Compilation": true,
 }
 
+// Precompiled regexes for NormalizeString — compiled once at package init.
+var (
+	bracketRe = regexp.MustCompile(`\[[^\]]*\]`)
+	parenRe   = regexp.MustCompile(`\([^)]*\)`)
+	yearRe    = regexp.MustCompile(`\b(1[0-9]{3}|2[0-9]{3})\b`)
+	spaceRe   = regexp.MustCompile(`\s+`)
+)
+
 // GetArtistReleaseGroups fetches all release groups for a given artist from MusicBrainz.
 // It queries the artist's release groups via the MusicBrainz Web Service API,
 // parses the XML response, and applies status and type filtering.
@@ -57,7 +65,9 @@ func (c *MusicBrainzClient) GetArtistReleaseGroups(ctx context.Context, artistMB
 		allGroups = append(allGroups, parsed.ReleaseGroups...)
 
 		// If we've fetched all results, we've reached the end.
-		if offset+len(parsed.ReleaseGroups) >= parsed.Count {
+		// Also break on empty page to prevent infinite loop if API
+		// returns fewer items than advertised by count.
+		if len(parsed.ReleaseGroups) == 0 || offset+len(parsed.ReleaseGroups) >= parsed.Count {
 			break
 		}
 
@@ -111,15 +121,12 @@ func NormalizeString(s string) string {
 	s = strings.ToLower(s)
 
 	// Remove bracketed content first (e.g., [Deluxe Edition], [Remastered 2020])
-	bracketRe := regexp.MustCompile(`\[[^\]]*\]`)
 	s = bracketRe.ReplaceAllString(s, "")
 
 	// Remove parenthesized content (e.g., (Deluxe), (Remastered))
-	parenRe := regexp.MustCompile(`\([^)]*\)`)
 	s = parenRe.ReplaceAllString(s, "")
 
 	// Remove years (4-digit numbers between 1000-2999)
-	yearRe := regexp.MustCompile(`\b(1[0-9]{3}|2[0-9]{3})\b`)
 	s = yearRe.ReplaceAllString(s, "")
 
 	// Replace common separators with spaces before stripping other special chars
@@ -136,7 +143,6 @@ func NormalizeString(s string) string {
 	s = b.String()
 
 	// Collapse multiple spaces
-	spaceRe := regexp.MustCompile(`\s+`)
 	s = spaceRe.ReplaceAllString(s, " ")
 
 	// Trim