feat(#112): reprise après refus — dossier complet, email resoumission

- GET/PATCH reprise-dossier enrichis (parents, enfants, motivation, fiche AM)
- Front: lien mail, modale identify login, wizards préremplis, PATCH complet
- Email accusé resoumission aux parents avec n° de dossier
- Fixes préremplissage AM (dates, places, ValueKey étape 2)

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

frontend/lib/screens/auth/parent_register_step3_screen.dart

@@ -166,7 +166,11 @@ class _ParentRegisterStep3ScreenState extends State<ParentRegisterStep3Screen> {
               if (await f.exists()) file = f;
             } catch (_) {}
           }
-          final updatedChild = oldChild.copyWith(imageBytes: bytes, imageFile: file);
+          final updatedChild = oldChild.copyWith(
+            imageBytes: bytes,
+            imageFile: file,
+            existingPhotoUrl: null,
+          );
           registrationData.updateChild(childIndex, updatedChild);
         }
       }
@@ -306,7 +310,7 @@ class _ParentRegisterStep3ScreenState extends State<ParentRegisterStep3Screen> {
                     onClearImage: () => setState(() {
                       final c = registrationData.children[index];
                       registrationData.updateChild(
-                          index, c.copyWith(imageFile: null, imageBytes: null));
+                          index, c.copyWith(imageFile: null, imageBytes: null, existingPhotoUrl: null));
                     }),
                     onDateSelect: () => _selectDate(context, index, registrationData),
                     onFirstNameChanged: (value) => setState(() {
@@ -414,7 +418,7 @@ class _ParentRegisterStep3ScreenState extends State<ParentRegisterStep3Screen> {
                                   onClearImage: () => setState(() {
                                     final c = registrationData.children[index];
                                     registrationData.updateChild(
-                                        index, c.copyWith(imageFile: null, imageBytes: null));
+                                        index, c.copyWith(imageFile: null, imageBytes: null, existingPhotoUrl: null));
                                   }),
                                   onDateSelect: () => _selectDate(context, index, registrationData),
                                   onFirstNameChanged: (value) => setState(() {