feat: admin notification system with priority ordering, bell icon, and /notifications page

Two-tier notification system: new_booking (all public bookings) + pending_booking (notes/today).
Priority-sorted queue, unread count polling, enriched responses with user_name/booking_start_time.
Fix critical bug: edit_requested cleanup was broken (wrong reason string in 3 handlers).
Add 15 new tests covering priority ordering, enrichment, and notification creation flows.
Update Admin Manual, Technical Manual, and gap backlog docs.

backend/handlers/notifications/notifications_test.go

@@ -199,7 +199,7 @@ func TestNotifications_ListFilterByReason(t *testing.T) {
 	}
 
 	// Create notifications with different reasons
-	reasons := []string{"pending_booking", "cancelled_booking", "edit_request"}
+	reasons := []string{"pending_booking", "cancelled_booking", "edit_requested"}
 	for _, reason := range reasons {
 		_, err := db.DB.Exec(context.Background(), `
 			INSERT INTO admin_notifications (reason, user_id)