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/routes/relais/dto/update-relais.dto.ts

@@ -0,0 +1,4 @@
+import { PartialType } from '@nestjs/swagger';
+import { CreateRelaisDto } from './create-relais.dto';
+
+export class UpdateRelaisDto extends PartialType(CreateRelaisDto) {}