feat(backend): implement Relais module and relation with Gestionnaire (Ticket #94)

- Create Relais entity
- Create Relais module, controller, service with CRUD
- Update Users entity with ManyToOne relation to Relais
- Update GestionnairesService to handle relaisId

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

backend/src/app.module.ts

@@ -16,6 +16,7 @@ import { AllExceptionsFilter } from './common/filters/all_exceptions.filters';
 import { EnfantsModule } from './routes/enfants/enfants.module';
 import { AppConfigModule } from './modules/config/config.module';
 import { DocumentsLegauxModule } from './modules/documents-legaux';
+import { RelaisModule } from './routes/relais/relais.module';
 
 @Module({
   imports: [
@@ -53,6 +54,7 @@ import { DocumentsLegauxModule } from './modules/documents-legaux';
     AuthModule,
     AppConfigModule,
     DocumentsLegauxModule,
+    RelaisModule,
   ],
   controllers: [AppController],
   providers: [