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-routing.php

@@ -99,6 +99,13 @@ class VC_Routing {
 			$term = get_queried_object();
 			if ( self::is_configured_term( $term ) ) {
 				$canonical = get_term_link( $term );
+				// Keep the page: /tour-destination/x/page/2/ and /x/?paged=2
+				// both land on /x/page/2/ (the form core's redirect_canonical
+				// would produce anyway).
+				$paged = (int) get_query_var( 'paged' );
+				if ( $paged > 1 && ! is_wp_error( $canonical ) ) {
+					$canonical = trailingslashit( $canonical ) . 'page/' . $paged . '/';
+				}
 			}
 		}
 		if ( ! $canonical || is_wp_error( $canonical ) ) {
@@ -114,6 +121,7 @@ class VC_Routing {
 		}
 		if ( isset( $parts[1] ) && '' !== $parts[1] ) {
 			$canonical .= '?' . $parts[1];
+			$canonical  = remove_query_arg( 'paged', $canonical );
 		}
 		wp_safe_redirect( $canonical, 301 );
 		exit;