fix(#131): co_parent en réponse parents + masquage secrets user

Contrat API fiche parent : co_parent peuplé (déjà chargé), sans password
ni tokens sur user/co_parent. Doc tmp front→back.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/routes/parents/parents.mapper.ts

@@ -0,0 +1,18 @@
+import { Parents } from 'src/entities/parents.entity';
+import { sanitizeUserForApi } from '../../common/utils/sanitize-user-for-api';
+
+/**
+ * Sérialisation API fiche parent — ticket #131.
+ * Garantit `user`, `co_parent` (si présent) et relations sans champs sensibles.
+ */
+export function mapParentForApi(parent: Parents): Parents {
+  return {
+    ...parent,
+    user: sanitizeUserForApi(parent.user)!,
+    co_parent: sanitizeUserForApi(parent.co_parent),
+  };
+}
+
+export function mapParentsForApi(parents: Parents[]): Parents[] {
+  return parents.map(mapParentForApi);
+}