Card validation improvements

[Harxhit]

Summary

• Add validation for description and visibility according to schema
• Validation level handling for empty platform links and duplicate links.
• Reject empty update payloads
• Improve validation error messages

Area

backend

[github-actions]

Hi @Harxhit,

Thanks for opening this issue.

Please reply with one of the following areas so the issue can be routed to the appropriate team member:

- /backend
- /web
- /mobile
- /devops

Once an area is selected, the corresponding label will be added automatically.

[Harxhit]

/backend

[github-actions]

The issue has been classified as backend.

@Harxhit, please review and triage this issue when available.

The backend label has been applied and the issue has been routed accordingly.