fix: paginated region/city pages, no routes before first save, safer structure save

- Add /{slug}/page/N/ rules and keep the page number through 301s
- Route only regions synced to a term, so unsaved presets never claim /europe/ etc.
- Flush rewrites on the request after activation so structure routes survive
- Suggest the existing term slug for new cities (keeps live /tour-destination/ URLs)
- Refuse saves without the editor sentinel; confirm before saving an empty structure
- Show term-sync failures to the admin instead of reporting success

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

viator-collector/assets/js/vc-structure-admin.js

@@ -43,7 +43,7 @@
 		var row = fromTemplate('vc-tpl-city');
 		row.querySelector('[data-f="id"]').value = item.id;
 		row.querySelector('[data-f="name"]').value = item.name;
-		row.querySelector('[data-f="slug"]').value = slugify(item.name);
+		row.querySelector('[data-f="slug"]').value = item.slug || slugify(item.name);
 		row.querySelector('.vc-city__name').textContent = item.name;
 		row.querySelector('.vc-city__id').textContent = item.id;
 		region.querySelector('.vc-cities tbody').appendChild(row);
@@ -147,8 +147,27 @@
 		region.querySelector('[data-f="name"]').focus();
 	});
 
-	form.addEventListener('submit', function () {
-		list.querySelectorAll('.vc-region').forEach(function (region, i) {
+	function hidden(name) {
+		var input = document.createElement('input');
+		input.type = 'hidden';
+		input.name = name;
+		input.value = '1';
+		form.appendChild(input);
+	}
+
+	form.addEventListener('submit', function (e) {
+		var regions = list.querySelectorAll('.vc-region');
+		if (!regions.length) {
+			if (!window.confirm(i18n.confirmEmpty)) {
+				e.preventDefault();
+				return;
+			}
+			hidden('vc_confirm_empty');
+		}
+		// Proves the editor ran: the server refuses a save without it, so a
+		// failed script can never submit an empty structure by accident.
+		hidden('vc_structure_present');
+		regions.forEach(function (region, i) {
 			region.querySelectorAll('.vc-region__head [data-f]').forEach(function (input) {
 				input.name = 'vc_structure[' + i + '][' + input.dataset.f + ']';
 			});