feat(notifications): complete notification delivery system with email, push, and in-app support

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>

prisma/schema.prisma

@@ -531,12 +531,14 @@ model NotificationLog {
   status      NotificationStatus @default(PENDING)
   errorDetail String?
   sentAt      DateTime?
+  readAt      DateTime?
   createdAt   DateTime           @default(now())
 
   @@index([userId])
   @@index([channel, status])
   @@index([templateKey])
   @@index([createdAt])
+  @@index([userId, readAt])
 }
 
 model NotificationPreference {