Validation

A record of cleaning up repeated validation and assignment logic inside GRIT's user profile initialization and update flow by moving the rules into the entity.

A note on why I introduced a PageRequestDto instead of exposing Spring Pageable directly, so that sort fields and page size rules could be controlled more clearly.

A note from implementing a gift API in Kakao Tech Campus, where I had to decide whether to rely only on database constraints or check some rules in the application first.