docs(backend): add swagger documentation

backend/src/user/dto/publicUser.dto.ts

@@ -1,4 +1,4 @@
-import { PickType } from "@nestjs/mapped-types";
+import { PickType } from "@nestjs/swagger";
 import { UserDTO } from "./user.dto";
 
 export class PublicUserDTO extends PickType(UserDTO, ["username"] as const) {}