fix: cover_url bug, add save-cover endpoint, fix svelte-check CI failure

- Fix handlers_image.go: cover_url now uses /api/cover/novelfire.net/{slug} (was 'local')
- Add POST /api/admin/image-gen/save-cover: persists pre-generated base64 directly to
  MinIO via PutCover without re-calling Cloudflare AI
- Add SvelteKit proxy route api/admin/image-gen/save-cover/+server.ts
- Update UI saveAsCover(): send existing image_b64 to save-cover instead of re-generating
- Run npm run paraglide to compile admin_nav_image_gen message; force-add gitignored files
- svelte-check: 0 errors, 21 warnings (all pre-existing)

backend/internal/backend/handlers_image.go

@@ -195,7 +195,7 @@ func (s *Server) handleAdminImageGen(w http.ResponseWriter, r *http.Request) {
 			// Non-fatal: still return the image
 		} else {
 			saved = true
-			coverURL = fmt.Sprintf("/api/cover/local/%s", req.Slug)
+			coverURL = fmt.Sprintf("/api/cover/novelfire.net/%s", req.Slug)
 			s.deps.Log.Info("admin: generated cover saved", "slug", req.Slug, "bytes", len(imgData))
 		}
 	}
@@ -213,6 +213,62 @@ func (s *Server) handleAdminImageGen(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// saveCoverRequest is the JSON body for POST /api/admin/image-gen/save-cover.
+type saveCoverRequest struct {
+	// Slug is the book slug whose cover should be overwritten.
+	Slug string `json:"slug"`
+	// ImageB64 is the base64-encoded image bytes (PNG or JPEG).
+	ImageB64 string `json:"image_b64"`
+}
+
+// handleAdminImageGenSaveCover handles POST /api/admin/image-gen/save-cover.
+//
+// Accepts a pre-generated image as base64 and stores it as the book cover in
+// MinIO, replacing the existing one. Does not call Cloudflare AI at all.
+func (s *Server) handleAdminImageGenSaveCover(w http.ResponseWriter, r *http.Request) {
+	if s.deps.CoverStore == nil {
+		jsonError(w, http.StatusServiceUnavailable, "cover store not configured")
+		return
+	}
+
+	var req saveCoverRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		jsonError(w, http.StatusBadRequest, "parse body: "+err.Error())
+		return
+	}
+	if req.Slug == "" {
+		jsonError(w, http.StatusBadRequest, "slug is required")
+		return
+	}
+	if req.ImageB64 == "" {
+		jsonError(w, http.StatusBadRequest, "image_b64 is required")
+		return
+	}
+
+	imgData, err := base64.StdEncoding.DecodeString(req.ImageB64)
+	if err != nil {
+		imgData, err = base64.RawStdEncoding.DecodeString(req.ImageB64)
+		if err != nil {
+			jsonError(w, http.StatusBadRequest, "decode image_b64: "+err.Error())
+			return
+		}
+	}
+
+	contentType := sniffImageContentType(imgData)
+	if err := s.deps.CoverStore.PutCover(r.Context(), req.Slug, imgData, contentType); err != nil {
+		s.deps.Log.Error("admin: save-cover failed", "slug", req.Slug, "err", err)
+		jsonError(w, http.StatusInternalServerError, "save cover: "+err.Error())
+		return
+	}
+
+	s.deps.Log.Info("admin: cover saved via image-gen", "slug", req.Slug, "bytes", len(imgData))
+	writeJSON(w, 0, map[string]any{
+		"saved":     true,
+		"cover_url": fmt.Sprintf("/api/cover/novelfire.net/%s", req.Slug),
+		"bytes":     len(imgData),
+	})
+}
+
 // sniffImageContentType returns the MIME type of the image bytes.
 func sniffImageContentType(data []byte) string {
 	if len(data) >= 4 {

backend/internal/backend/server.go

@@ -189,6 +189,7 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	// Admin image generation endpoints
 	mux.HandleFunc("GET /api/admin/image-gen/models", s.handleAdminImageGenModels)
 	mux.HandleFunc("POST /api/admin/image-gen", s.handleAdminImageGen)
+	mux.HandleFunc("POST /api/admin/image-gen/save-cover", s.handleAdminImageGenSaveCover)
 
 	// Voices list
 	mux.HandleFunc("GET /api/voices", s.handleVoices)