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/api/router.go

@@ -43,6 +43,11 @@ func NewRouter(h *Handlers) *chi.Mux {
 
 		r.Post("/sync", h.Sync)
 
+		r.Route("/artists", func(r chi.Router) {
+			r.Post("/{id}/refresh", h.RefreshArtist)
+			r.Delete("/{id}", h.DeleteArtist)
+		})
+
 		r.Route("/library", func(r chi.Router) {
 			r.Get("/artists", h.ListLibraryArtists)
 			r.Get("/albums", h.ListLibraryAlbums)