Add 5 new event listeners (listing.approved, listing.rejected, payment.confirmed,
subscription.expiring, inquiry.received), 3 new Handlebars templates, readAt field
for in-app read/unread tracking, unread/mark-as-read API endpoints, and unit tests.
All 57 notification tests pass, lint clean, typecheck clean.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
