fix: reject snapshot when template already attached (409); handle domains-load error; drop orphaned useDeleteDomain

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/api/handlers.go

@@ -114,6 +114,16 @@ func (a *API) handleTemplateFromZone(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, http.StatusNotFound, "домен не найден")
 		return
 	}
+	// This endpoint only makes sense for a domain with no template attached
+	// yet — it snapshots the zone's live state into a brand-new template.
+	// If a template is already bound, re-attaching a fresh snapshot would
+	// silently orphan the existing one; re-pointing a domain to a different
+	// template is a separate, explicit action and must not happen as a side
+	// effect of a retried/duplicate POST here.
+	if dom.TemplateID != nil {
+		writeErr(w, http.StatusConflict, "шаблон уже привязан")
+		return
+	}
 	recs, err := a.Svc.ZoneRecords(r.Context(), pid, did)
 	if err != nil {
 		log.Printf("api: template-from-zone: zone records failed: %v", err)