feat(#131): back placement AM↔enfant + BDD garde/sans_garde

- API PATCH …/fiche, POST/DELETE …/enfants/:enfantId, GET avec amChildren
- Table enfants_assistantes_maternelles (option D, 1 garde active/enfant)
- Statuts enfant: garde/sans_garde remplacent actif (inscription → sans_garde)
- BDD.sql canonique réécrit; migration pour BDD existantes
- Seed test: hash bcrypt corrigé pour mot de passe « password »

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

backend/src/entities/am_children.entity.ts

@@ -0,0 +1,47 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  ManyToOne,
+  JoinColumn,
+  CreateDateColumn,
+} from 'typeorm';
+import { AssistanteMaternelle } from './assistantes_maternelles.entity';
+import { Children } from './children.entity';
+import { Users } from './users.entity';
+
+@Entity('enfants_assistantes_maternelles', { schema: 'public' })
+export class AmChildren {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ name: 'id_am', type: 'uuid' })
+  amId: string;
+
+  @Column({ name: 'id_enfant', type: 'uuid' })
+  enfantId: string;
+
+  @Column({ name: 'date_debut', type: 'date' })
+  date_debut: Date;
+
+  @Column({ name: 'date_fin', type: 'date', nullable: true })
+  date_fin?: Date;
+
+  @CreateDateColumn({ name: 'cree_le', type: 'timestamptz' })
+  cree_le: Date;
+
+  @Column({ name: 'cree_par', type: 'uuid', nullable: true })
+  cree_par?: string;
+
+  @ManyToOne(() => AssistanteMaternelle, (am) => am.amChildren, { onDelete: 'CASCADE' })
+  @JoinColumn({ name: 'id_am', referencedColumnName: 'user_id' })
+  am: AssistanteMaternelle;
+
+  @ManyToOne(() => Children, (c) => c.amLinks, { onDelete: 'CASCADE' })
+  @JoinColumn({ name: 'id_enfant', referencedColumnName: 'id' })
+  child: Children;
+
+  @ManyToOne(() => Users, { nullable: true, onDelete: 'SET NULL' })
+  @JoinColumn({ name: 'cree_par', referencedColumnName: 'id' })
+  createdBy?: Users;
+}