fix(backend): automatically handle default card reassignment on card deletion

[mramansayyad]

Summary

Prevents database inconsistencies and profile breakage by restricting or gracefully handling the deletion of a user's active default profile card.

Contexts

Currently, the DELETE /cards/:id endpoint allows a user to delete any card belonging to them. If they delete their active default card (isDefault: true), the user profile is left without a default fallback state. This breaks core external entry points, such as primary QR code resolution and the public-facing web backup route (apps/web), which depend on a valid default card.

Tasks

• Intercept DELETE /cards/:id requests to check if the target card has isDefault: true.
• Implement fallback logic: if other cards exist, automatically promote the oldest remaining card to isDefault: true.
• Prevent deletion or return a descriptive error if it is the user's last remaining card (or auto-create a baseline fallback).
• Add automated unit and integration tests covering the new deletion behavior.

Acceptance Criteria

• Attempting to delete a default card gracefully reassigns the default flag to an existing sibling card.
• Deleting a non-default card leaves the default card unaffected.
• Back-end unit tests added and pass cleanly (pnpm test:backend).

Area

backend

Difficulty

Medium

[pari7maheshwari]

hey @mramansayyad I would like to work on this issue under gssoc. Can u please assign it to me

[Harxhit]

@mramansayyad I have assigned this issue to you.