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

testing/e2e/testutil/testutil.go

@@ -183,6 +183,11 @@ func (e *TestEnv) GET(path string) (*APIResponse, error) {
 	return e.Do(APIRequest{Method: http.MethodGet, Path: path})
 }
 
+// DELETE is a convenience method for DELETE requests.
+func (e *TestEnv) DELETE(path string) (*APIResponse, error) {
+	return e.Do(APIRequest{Method: http.MethodDelete, Path: path})
+}
+
 // DecodeJSON decodes the response body into the given value.
 func (r *APIResponse) DecodeJSON(v any) error {
 	return json.Unmarshal(r.Body, v)
@@ -244,6 +249,26 @@ func (e *TestEnv) GetArtistByForeignID(ctx context.Context, foreignID string) (m
 	return result, nil
 }
 
+// GetArtistUpdatedAt retrieves the updated_at timestamp for an artist.
+func (e *TestEnv) GetArtistUpdatedAt(ctx context.Context, foreignArtistID string) (time.Time, error) {
+	var updatedAt time.Time
+	err := e.DB.QueryRow(ctx, `
+		SELECT updated_at FROM artist_metadata WHERE foreign_artist_id = $1
+	`, foreignArtistID).Scan(&updatedAt)
+	return updatedAt, err
+}
+
+// CountAlbumsByArtist returns the number of albums for a specific artist.
+func (e *TestEnv) CountAlbumsByArtist(ctx context.Context, foreignArtistID string) (int64, error) {
+	var count int64
+	err := e.DB.QueryRow(ctx, `
+		SELECT COUNT(*) FROM albums a
+		JOIN artist_metadata am ON a.artist_metadata_id = am.id
+		WHERE am.foreign_artist_id = $1
+	`, foreignArtistID).Scan(&count)
+	return count, err
+}
+
 // GetAlbumsByArtistForeignID retrieves albums for an artist by foreign artist ID.
 func (e *TestEnv) GetAlbumsByArtistForeignID(ctx context.Context, foreignArtistID string) ([]map[string]any, error) {
 	rows, err := e.DB.Query(ctx, `