Agregar campo username a User entity y DTO

- Columna username (unique, nullable) en auth.users
- Campo username en CreateUserDto y UpdateUserDto

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/modules/users/dto/create-user.dto.ts

@@ -18,6 +18,7 @@ export class CreateUserDto {
   @ApiProperty({ description: 'Last name', example: 'Doe' })
   @IsString()
   lastName: string;
+@ApiPropertyOptional({ description: 'Username', example: 'johndoe' })  @IsOptional()  @IsString()  username?: string;
 
   @ApiPropertyOptional({ description: 'Phone number', example: '+1234567890' })
   @IsOptional()