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/auth/dto/enfant-inscription.dto.ts

@@ -59,5 +59,14 @@ export class EnfantInscriptionDto {
   @IsOptional()
   @IsBoolean()
   grossesse_multiple?: boolean;
+
+  @ApiProperty({
+    example: true,
+    required: false,
+    description: 'Consentement affichage / stockage photo (colonne enfants.consentement_photo)',
+  })
+  @IsOptional()
+  @IsBoolean()
+  consent_photo?: boolean;
 }