FluentValidation for Post/Category DTOs

[unrealbg]

Problem

No strong validation rules for Post/Category DTOs; UI does not show errors inline.

Proposal

• Add server-side validation via FluentValidation.
• Show validation messages inline in the client UI.
• Share rules between client and server for consistency.

Alternatives considered

• Manual validation only.

Acceptance criteria

• Server-side validation via FluentValidation
• Client UI shows messages inline
• Rules shared/consistent between client and server

Technical notes

• Validators for create/update DTOs
• Integrate with EditForm and ValidationMessage

Risks

• Inconsistent validation if rules diverge between client and server.

Additional context

Labels: enhancement, good first issue