Phase 6 refactor

[nkcoder]

What

1. Optional Return Types: Optional communicates that absence is expected; orThrow methods
   for when null is a bug
2. Method References: userMapper::toResponseOrThrow is cleaner than user ->
   userMapper.toResponseOrThrow(user)
3. Functional Chains: repository.findById(id).map(...).orElseThrow(...) is more readable than
   nested if-else
4. Constants Classes: Centralize messages for consistency, easier i18n, and preventing typos
5. Compact Constructors: Normalize data (lowercase email, trim strings) at construction time
6. Static Imports: import static ...ValidationMessages.* keeps code readable with constants