feat: add user operations to backend

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

@@ -1,17 +1,25 @@
 import { Expose, plainToClass } from "class-transformer";
-import { IsEmail, IsNotEmpty, IsOptional, IsString } from "class-validator";
+import {
+  IsEmail,
+  IsNotEmpty,
+  IsString,
+  Length,
+  Matches,
+} from "class-validator";
 
 export class UserDTO {
   @Expose()
   id: string;
 
   @Expose()
-  @IsOptional()
-  @IsString()
+  @Expose()
+  @Matches("^[a-zA-Z0-9_.]*$", undefined, {
+    message: "Username can only contain letters, numbers, dots and underscores",
+  })
+  @Length(3, 32)
   username: string;
 
   @Expose()
-  @IsOptional()
   @IsEmail()
   email: string;
 
@@ -25,4 +33,10 @@ export class UserDTO {
   from(partial: Partial<UserDTO>) {
     return plainToClass(UserDTO, partial, { excludeExtraneousValues: true });
   }
+
+  fromList(partial: Partial<UserDTO>[]) {
+    return partial.map((part) =>
+      plainToClass(UserDTO, part, { excludeExtraneousValues: true })
+    );
+  }
 }