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/migrations/20260409000000_add_notification_read_at/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "NotificationLog" ADD COLUMN "readAt" TIMESTAMP(3);
+
+-- CreateIndex
+CREATE INDEX "NotificationLog_userId_readAt_idx" ON "NotificationLog"("userId", "readAt");