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/includes/class-vc-settings.php

@@ -63,6 +63,33 @@ class VC_Settings {
 			}
 		}
 
+		// Suggested URL slug for the Viator Structure screen: keep an existing
+		// term's slug so live /tour-destination/{slug}/ links still resolve
+		// (and 301), otherwise WordPress's own slug for the name.
+		$existing = array();
+		if ( $results ) {
+			$terms = get_terms(
+				array(
+					'taxonomy'   => 'viator_destination',
+					'hide_empty' => false,
+					'meta_query' => array( // phpcs:ignore WordPress.DB.SlowDBQuery
+						array(
+							'key'     => '_vc_viator_destination_id',
+							'value'   => array_column( $results, 'id' ),
+							'compare' => 'IN',
+						),
+					),
+				)
+			);
+			foreach ( is_wp_error( $terms ) ? array() : $terms as $term ) {
+				$existing[ (string) get_term_meta( $term->term_id, '_vc_viator_destination_id', true ) ] = $term->slug;
+			}
+		}
+		foreach ( $results as &$result ) {
+			$result['slug'] = isset( $existing[ $result['id'] ] ) ? $existing[ $result['id'] ] : sanitize_title( $result['name'] );
+		}
+		unset( $result );
+
 		wp_send_json_success( array( 'results' => $results ) );
 	}