Add Popular Routes

Curated from/to destination pairs for marketing content, editable via a
Filament resource and served publicly (no auth, IP-throttled like CMS
pages) through GET /api/v1/popular-routes. The API response trims each
nested destination down to id/name/mm_name rather than the full catalog
resource.

app-modules/routing/src/Observers/PopularRouteObserver.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Modules\Routing\Observers;
+
+use Illuminate\Support\Facades\Cache;
+use Modules\Routing\Models\PopularRoute;
+
+class PopularRouteObserver
+{
+    public function saved(PopularRoute $popularRoute): void
+    {
+        Cache::tags('routes')->flush();
+    }
+
+    public function deleted(PopularRoute $popularRoute): void
+    {
+        Cache::tags('routes')->flush();
+    }
+}