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/handlers.go

@@ -269,6 +269,60 @@ func (h *Handlers) LibraryStats(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func (h *Handlers) RefreshArtist(w http.ResponseWriter, r *http.Request) {
+	if h.DB == nil {
+		writeError(w, http.StatusServiceUnavailable, "database not connected")
+		return
+	}
+
+	artistID := chi.URLParam(r, "id")
+	if artistID == "" {
+		writeError(w, http.StatusBadRequest, "artist ID required")
+		return
+	}
+
+	result, err := services.RefreshArtist(r.Context(), artistID, h.MetadataClient, h.DB)
+	if err != nil {
+		if _, ok := err.(*services.NotFoundError); ok {
+			writeError(w, http.StatusNotFound, err.Error())
+			return
+		}
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, result)
+}
+
+func (h *Handlers) DeleteArtist(w http.ResponseWriter, r *http.Request) {
+	if h.DB == nil {
+		writeError(w, http.StatusServiceUnavailable, "database not connected")
+		return
+	}
+
+	artistID := chi.URLParam(r, "id")
+	if artistID == "" {
+		writeError(w, http.StatusBadRequest, "artist ID required")
+		return
+	}
+
+	deleted, err := h.DB.DeleteArtistByForeignID(r.Context(), artistID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	if !deleted {
+		writeError(w, http.StatusNotFound, "artist not found: "+artistID)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"deleted": true,
+		"message": "artist and related data deleted",
+	})
+}
+
 func writeJSON(w http.ResponseWriter, status int, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)