Create UserMapper in Application Layer

[gorushkin]

Type: Refactoring | Priority: Medium | Depends on: REFACTOR-001

Description:

After removing DB and DTO methods from User entity, the transformation logic must live in an application-layer mapper (application/mappers/user.mapper.ts). Currently this logic is scattered across use cases and middleware.

Tasks:

1. Create application/mappers/user.mapper.ts with:
   • toResponseDTO(snapshot: UserSnapshot): UserResponseDTO
   • toDomain(row: UserDbRow): User (replaces User.fromPersistence())
   • toDBRow(snapshot: UserSnapshot): UserDbInsert (replaces user.toPersistence())
2. Update loginUser.ts and registerUser.ts to use UserMapper.toResponseDTO()
3. Update loginUser.ts and auth.middleware.ts to use UserMapper.toDomain() instead of User.fromPersistence()
4. Register UserMapper in di/container.ts

Acceptance Criteria:

• No use case or middleware calls user.toPersistence(), user.toResponseDTO(), or User.fromPersistence()
• UserMapper covers all three transformation directions
• TypeScript check passes (pnpm --filter backend ts-check)