feat: add refresh and delete artist endpoints (sections 1.2, 1.3)

- Add POST /api/artists/{id}/refresh to re-fetch metadata from gRPC service
- Add DELETE /api/artists/{id} with cascade delete via PostgreSQL
- Add e2e tests for both flows covering happy path, not found, idempotency
- Extend testutil with GetArtistUpdatedAt, CountAlbumsByArtist, DELETE helper

internal/database/db.go

@@ -250,3 +250,41 @@ func (db *DB) CountAlbums(ctx context.Context) (int64, error) {
 	err := db.pool.QueryRow(ctx, "SELECT COUNT(*) FROM albums").Scan(&count)
 	return count, err
 }
+
+func (db *DB) GetArtistByForeignID(ctx context.Context, foreignArtistID string) (*ArtistMetadataRow, error) {
+	var a ArtistMetadataRow
+	err := db.pool.QueryRow(ctx, `
+		SELECT id, foreign_artist_id, name, sort_name, artist_type, genres, created_at, updated_at
+		FROM artist_metadata
+		WHERE foreign_artist_id = $1
+	`, foreignArtistID).Scan(&a.ID, &a.ForeignArtistID, &a.Name, &a.SortName, &a.ArtistType, &a.Genres, &a.CreatedAt, &a.UpdatedAt)
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+func (db *DB) CountAlbumsByArtist(ctx context.Context, artistMetadataID uuid.UUID) (int64, error) {
+	var count int64
+	err := db.pool.QueryRow(ctx, `
+		SELECT COUNT(*) FROM albums WHERE artist_metadata_id = $1
+	`, artistMetadataID).Scan(&count)
+	return count, err
+}
+
+func (db *DB) TouchArtistUpdatedAt(ctx context.Context, artistMetadataID uuid.UUID) error {
+	_, err := db.pool.Exec(ctx, `
+		UPDATE artist_metadata SET updated_at = NOW() WHERE id = $1
+	`, artistMetadataID)
+	return err
+}
+
+func (db *DB) DeleteArtistByForeignID(ctx context.Context, foreignArtistID string) (bool, error) {
+	result, err := db.pool.Exec(ctx, `
+		DELETE FROM artist_metadata WHERE foreign_artist_id = $1
+	`, foreignArtistID)
+	if err != nil {
+		return false, err
+	}
+	return result.RowsAffected() > 0, nil
+}