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/admin/bookings_test.go

@@ -1720,7 +1720,7 @@ func TestAdminBookings_ApproveEditRequest(t *testing.T) {
 	// Create admin notification
 	_, err = db.DB.Exec(context.Background(),
 		`INSERT INTO admin_notifications (reason, booking_id, user_id)
-		VALUES ('edit_request', $1, $2)`,
+		VALUES ('edit_requested', $1, $2)`,
 		bookingID, userID)
 	if err != nil {
 		t.Fatalf("failed to create admin notification: %v", err)
@@ -1773,7 +1773,7 @@ func TestAdminBookings_ApproveEditRequest(t *testing.T) {
 	var ackTime *time.Time
 	err = db.DB.QueryRow(context.Background(),
 		`SELECT acknowledged_at FROM admin_notifications
-		WHERE booking_id = $1 AND reason = 'edit_request'`,
+		WHERE booking_id = $1 AND reason = 'edit_requested'`,
 		bookingID).Scan(&ackTime)
 	if err != nil {
 		t.Fatalf("failed to query notification: %v", err)