feat(inscription): payload enfant/photo et limite JSON 15 Mo côté API

- Express/Nest : body parser json et urlencoded à 15 Mo pour les photos base64.
- Front : envoi d’inscription parent (authService, payload, étape 3).

Made-with: Cursor

frontend/lib/screens/auth/parent_register_step3_screen.dart

@@ -149,7 +149,11 @@ class _ParentRegisterStep3ScreenState extends State<ParentRegisterStep3Screen> {
     final ImagePicker picker = ImagePicker();
     try {
       final XFile? pickedFile = await picker.pickImage(
-        source: ImageSource.gallery, imageQuality: 70, maxWidth: 1024, maxHeight: 1024);
+        source: ImageSource.gallery,
+        imageQuality: 60,
+        maxWidth: 900,
+        maxHeight: 900,
+      );
       if (pickedFile != null) {
         if (childIndex < registrationData.children.length) {
           final oldChild = registrationData.children[childIndex];