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>

backend/src/routes/enfants/enfants.service.ts

@@ -120,7 +120,13 @@ export class EnfantsService {
     const child = await this.childrenRepository.findOne({ where: { id } });
     if (!child) throw new NotFoundException('Enfant introuvable');
 
-    await this.childrenRepository.update(id, dto);
+    const patch: Partial<Children> = { ...dto } as Partial<Children>;
+    if (dto.consent_photo !== undefined) {
+      patch.consent_photo = dto.consent_photo;
+      patch.consent_photo_at = dto.consent_photo ? new Date() : null!;
+    }
+
+    await this.childrenRepository.update(id, patch);
     return this.findOne(id, currentUser);
   }