Clarify persisted cover handling

backend/internal/web/cover.go

@@ -9,9 +9,9 @@ import (
 )
 
 // CoverFetcher retrieves one kagane cover by image id. Satisfied by
-// latest.BrowserFetcher, and nil when BROWSER_WS_URL is unset — which leaves
-// kagane covers exactly as unavailable as they were before this endpoint
-// existed, rather than hanging a request on a fetcher that cannot run.
+// latest.BrowserFetcher. It is nil when BROWSER_WS_URL is unset; uncached
+// covers are then unavailable, while covers already stored by the backend
+// remain available without a browser.
 type CoverFetcher interface {
 	Image(ctx context.Context, imageID string) (body []byte, contentType string, err error)
 }
@@ -52,11 +52,13 @@ func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if body, contentType, ok, err := h.store.GetKaganeCover(id); err != nil {
+	body, contentType, ok, err := h.store.GetKaganeCover(id)
+	if err != nil {
 		log.Printf("read kagane cover %s: %v", id, err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
 		return
-	} else if ok {
+	}
+	if ok {
 		writeCover(w, body, contentType)
 		return
 	}
@@ -67,7 +69,7 @@ func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
 
 	ctx, cancel := context.WithTimeout(r.Context(), coverTimeout)
 	defer cancel()
-	body, contentType, err := h.covers.Image(ctx, id)
+	body, contentType, err = h.covers.Image(ctx, id)
 	if err != nil {
 		log.Printf("kagane cover %s: %v", id, err)
 		http.NotFound(w, r)