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/user/dto/create_gestionnaire.dto.ts

@@ -1,4 +1,10 @@
-import { OmitType } from "@nestjs/swagger";
+import { ApiProperty, OmitType } from "@nestjs/swagger";
 import { CreateUserDto } from "./create_user.dto";
+import { IsOptional, IsUUID } from "class-validator";
 
-export class CreateGestionnaireDto extends OmitType(CreateUserDto, ['role'] as const) {}
+export class CreateGestionnaireDto extends OmitType(CreateUserDto, ['role'] as const) {
+  @ApiProperty({ required: false, description: 'ID du relais de rattachement' })
+  @IsOptional()
+  @IsUUID()
+  relaisId?: string;
+}