feat(api): validate kind on PUT /bookmarks

backend/internal/api/handlers.go

@@ -78,6 +78,15 @@ func (h *Handler) Put(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Same rule as status: empty means "keep the stored value". An unknown
+	// value is a client bug, not something to silently coerce to manga.
+	switch b.Kind {
+	case "", store.KindManga, store.KindNovel:
+	default:
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "invalid kind"})
+		return
+	}
+
 	// Candidate timestamp, not a decision: Upsert keeps the stored one unless
 	// reading progress actually moved. Any client value is ignored.
 	b.UpdatedAt = time.Now().UnixMilli()