Add priority expiry field to attendee schema

[aniketgargya]

Initial commit to ensure expiry field actually exists on the attendee document

TODO:

• Change priority expiry when registering an attendee to null if it's a "negative" event, or to Date.now() + 24 hours if it's a "positive" event
• Add priority status to /auth/me
• Return priority status when scanning an attendee's QR code

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	63.59% (+0.05% 🔼)	496/780
🔴	Branches	14%	14/100
🔴	Functions	27.36%	29/106
🟡	Lines	61.36% (+0.05% 🔼)	432/704

Test suite run success

24 tests passing in 15 suites.

Report generated by 🧪jest coverage report action from aeb16d4ef26d6043106d6b86a1bd746e8f4eda36

[xuxey]

@aniketgargya Before merging, please rebase onto main & bump the minor verison in app.service.ts