fix(cards): FK card_responses via relation JoinColumn + save(create)

Double mapping @Column/@JoinColumn sur id_card ; persist response via
card/user relations. Garde update() sur l’instance (évite save parent
qui nullifiait les FK enfants).

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

backend/src/entities/card_responses.entity.ts

@@ -5,6 +5,7 @@ import {
   JoinColumn,
   ManyToOne,
   PrimaryGeneratedColumn,
+  RelationId,
 } from 'typeorm';
 import { CardInstance } from './card_instances.entity';
 import { Users } from './users.entity';
@@ -20,20 +21,21 @@ export class CardResponse {
   @PrimaryGeneratedColumn('uuid')
   id: string;
 
-  @Column({ name: 'id_card', type: 'uuid' })
-  id_card: string;
-
+  /** FK carte — une seule source de vérité via @JoinColumn (pas de @Column doublon). */
   @ManyToOne(() => CardInstance, (c) => c.responses, { onDelete: 'CASCADE' })
   @JoinColumn({ name: 'id_card', referencedColumnName: 'id' })
   card: CardInstance;
 
-  @Column({ name: 'id_utilisateur', type: 'uuid' })
-  id_utilisateur: string;
+  @RelationId((r: CardResponse) => r.card)
+  id_card: string;
 
   @ManyToOne(() => Users, { onDelete: 'CASCADE' })
   @JoinColumn({ name: 'id_utilisateur', referencedColumnName: 'id' })
   user: Users;
 
+  @RelationId((r: CardResponse) => r.user)
+  id_utilisateur: string;
+
   @Column({
     type: 'enum',
     enum: CardResponseActionType,