fix: address code review findings

- Fix pagination to not trust MusicBrainz count for cutoff (prevents
  silent discography truncation / hidden missing releases)
- Add idx_external_releases_artist_id index for hot per-artist queries
- Dedupe cached_at layout constant in external_releases.go

internal/musicbrainz/api.go

@@ -46,10 +46,14 @@ func (c *MusicBrainzClient) GetArtistReleaseGroups(ctx context.Context, artistMB
 
 		allGroups = append(allGroups, parsed.ReleaseGroups...)
 
-		// If we've fetched all results, we've reached the end.
-		// 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 {
+		// Stop when a page is empty (no more results) or when the page
+		// returned fewer items than the request limit — a reliable end-of-data
+		// signal. We intentionally do NOT trust parsed.Count for the cutoff:
+		// MusicBrainz occasionally reports an inaccurate count, which would
+		// prematurely truncate an artist's discography and hide missing
+		// releases. The empty-page check also prevents an infinite loop if the
+		// API keeps returning a non-empty page past the reported count.
+		if len(parsed.ReleaseGroups) == 0 || len(parsed.ReleaseGroups) < limit {
 			break
 		}