DTO validation with jakarta.validation and @Valid in controllers

[sugan0tech]

• Description: Add validation annotations to DTOs and enforce with @Valid.
• Current state: DTOs like UserSigninDTO, CreateElementRequest, CreateAvatarRequest lack validation; many controllers accept Map instead of typed DTOs.
• Tasks:
  • Add dependency spring-boot-starter-validation and annotate DTO fields (@NotBlank, @Size, @Positive, etc.).
  • Change controller signatures to use typed DTOs and @Valid.
  • Provide meaningful validation messages.
• Acceptance criteria:
  • Invalid payloads yield 400 with field errors through the global handler.
• References: DTOs in features/*, multiple controllers (e.g., AuthController, SpaceController).