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/database/database_test.go

@@ -205,9 +205,10 @@ func TestMigrationTracking(t *testing.T) {
 		t.Fatalf("query migrations count: %v", err)
 	}
 
-	// We have 6 recorded migrations: artist_settings, external_releases,
-	// local_albums, notifications_sent, cached_at column, secondary_types column.
-	if count != 6 {
-		t.Errorf("expected 6 applied migrations, got %d", count)
+	// We have 7 recorded migrations: artist_settings, external_releases,
+	// local_albums, notifications_sent, cached_at column, secondary_types
+	// column, and the external_releases.artist_id index.
+	if count != 7 {
+		t.Errorf("expected 7 applied migrations, got %d", count)
 	}
 }