fix(#144): persister le consentement photo des enfants

- DTO inscription enfant : champ consent_photo
- Inscription + reprise parent : sauvegarde bool + date
- Front payload : envoi consent_photo
- PATCH /enfants : horodatage du consentement

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

frontend/lib/utils/parent_registration_payload.dart

@@ -167,6 +167,7 @@ class ParentRegistrationPayload {
     final map = <String, dynamic>{
       'genre': apiGenres.contains(c.genre) ? c.genre : 'Autre',
       'grossesse_multiple': c.multipleBirth,
+      'consent_photo': c.photoConsent,
     };
 
     final prenom = c.firstName.trim();