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.go

@@ -121,6 +121,10 @@ func (db *DB) migrate() error {
 			name: "006_add_secondary_types_to_external_releases",
 			sql:  `ALTER TABLE external_releases ADD COLUMN secondary_types TEXT;`,
 		},
+		{
+			name: "007_index_external_releases_artist_id",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_external_releases_artist_id ON external_releases(artist_id);`,
+		},
 	}
 
 	for _, m := range migrations {

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)
 	}
 }

internal/database/external_releases.go

@@ -175,11 +175,10 @@ func GetExternalReleasesByArtistWithCache(db *DB, artistID string, ttl time.Dura
 	if ttl <= 0 {
 		return nil, nil
 	}
-	// cached_at is stored in the "2006-01-02 15:04:05" UTC layout via
-	// FormatCachedAt. Compare against an explicitly formatted cutoff string in
-	// the same layout so the lexicographic comparison is a valid time ordering.
-	const layout = "2006-01-02 15:04:05"
-	cutoff := time.Now().UTC().Add(-ttl).Format(layout)
+	// cached_at is stored in utcLayout via FormatCachedAt. Compare against an
+	// explicitly formatted cutoff string in the same layout so the
+	// lexicographic comparison is a valid time ordering.
+	cutoff := time.Now().UTC().Add(-ttl).Format(utcLayout)
 	rows, err := db.Conn().Query(
 		"SELECT rgid, artist_id, title, type, release_date, is_ignored, cached_at, secondary_types FROM external_releases WHERE artist_id = ? AND cached_at >= ?",
 		artistID, cutoff,