fix(backend): wrap card updates in a transaction to prevent data loss

[anshul23102]

Closes #216

[Prince2301p]

Hi,
I would like to work on resolving this issue. Please assign it to me. I’m a participant in GSSoC 2026 and would be happy to contribute.
Looking forward to collaborating with you.

[anshul23102]

Hi @Dev-Card team, just following up on this one. The issue it addresses is a silent data loss scenario: if any step in the card link update fails midway, partial writes get committed and the card ends up in an inconsistent state with no way to know which links were saved and which were not. Wrapping the whole operation in a Prisma transaction ensures it either fully succeeds or fully rolls back. All existing tests pass. Let me know if you would like any changes.

[anshul23102]

Hi @PrashantkumarKhatri! The Vercel check is showing 'Authorization required to deploy' on this one too - could you authorize the preview deployment when you get a chance? Ready for review whenever you are. Thanks!