feat: sprint 3 — profile endpoints + avatar

- GET /api/auth/me enrichi : avatar, plan actif, subscriptionDate
- GET /api/users/me/profile : profil complet (local UUID, sub, rôles)
- PATCH /api/users/me : update nickname / avatar (validation URL + longueur)
- User entity : champ avatar VARCHAR(500) nullable
- Migration 1742000000000-AddUserAvatar (appliquée VPS)

backend/src/entities/User.ts

@@ -22,6 +22,9 @@ export class User {
   @Column({ type: "varchar", length: 100 })
   nickname!: string;
 
+  @Column({ type: "varchar", length: 500, nullable: true })
+  avatar!: string | null;
+
   @Column({ type: "boolean", default: true })
   isActive!: boolean;