Add QR code generation for event check-in #670

Feature Request

Is your feature request related to a problem?

Event organizers currently handle attendance manually (paper sheets or roll-calls). This is slow, error-prone, and produces no digital record. There is no quick way for attendees to confirm their presence.

Describe the solution you'd like

When an event is created, automatically generate a unique QR code linking to a check-in endpoint (/events/:id/checkin?token=<unique_token>). Organizers can display this code at the venue or share it digitally. Attendees scan it to register their attendance.

Use a library such as qrcode (Node) or qrcode.react (React) for generation:

import QRCode from 'qrcode';
const qrDataUrl = await QRCode.toDataURL(`https://uni-event.app/events/${eventId}/checkin?token=${token}`);

Store check-in records with userId, eventId, and timestamp. Show a live attendee count on the organizer dashboard.

Describe alternatives you've considered

• Manual attendance form (no digital record, slow)
• Third-party check-in apps (requires additional accounts and breaks the integrated experience)

Additional context

Level 2 feature. QR codes should expire after the event end time to prevent misuse.