fix: remove duplicate tenant service method and handle not found

backend/internal/api/handlers/tenant.go

@@ -56,15 +56,14 @@ func (h *TenantHandler) CheckExists(w http.ResponseWriter, r *http.Request) {
 
 	tenant, err := h.tenantService.GetBySubdomain(subdomain)
 	if err != nil {
+		if err == service.ErrTenantNotFound {
+			http.NotFound(w, r)
+			return
+		}
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
 		return
 	}
 
-	if tenant == nil {
-		http.NotFound(w, r)
-		return
-	}
-
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
 }