feat(#152): suppression complète grossesse multiple / est_multiple.

Retire le champ partout : BDD (migration + seed), entity/DTO/services Nest,
modèles et payloads Flutter, scripts de test et docs. Back et front alignés
(forbidNonWhitelisted). Aucun fantôme de compat.

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

backend/prisma/schema.prisma

@@ -33,7 +33,6 @@ model Child {
   dateOfBirth   DateTime
   photoUrl      String?
   photoConsent  Boolean   @default(false)
-  isMultiple    Boolean   @default(false)
   isUnborn      Boolean   @default(false)
   parentId      String
   parent        Parent    @relation(fields: [parentId], references: [id])