backend: implement NFC tag payload generation endpoint

[ShantKhatri]

Summary

Product doc section 5.2 specifies NFC tap support — a physical card or badge sticker programmed with the DevCard link. Add a backend endpoint that generates and returns the correct NDEF payload (NFC Data Exchange Format) for a user's DevCard URL so the mobile app can write it to an NFC tag.

Context

This is a post-MVP feature but the backend work is self-contained and maps to apps/backend/src/routes/ as a new nfc.ts route file.

Tasks

• create apps/backend/src/routes/nfc.ts.
• add GET /api/nfc/payload (authenticated) that returns:

  { "type": "URI", "payload": "https://devcard.dev/<username>" }

• add GET /api/nfc/payload?card=<cardId> variant for context card-specific URLs.
• validate that cardId (if provided) belongs to the authenticated user.
• register route in apps/backend/src/app.ts.
• add tests asserting the correct payload structure and ownership check.
• document the endpoint in apps/backend/README.md with a note linking to the ndefjs library the mobile team should use on the client side.

Acceptance Criteria

• endpoint returns correct NDEF-compatible payload.
• card ownership is validated before returning the card-specific URL.
• tests pass.

Difficulty

medium — straightforward route design but requires understanding NDEF URI record structure.