feat(global): add public API endpoint for navigation data

Expose primaryNav, footerLinks, and socialMediaLinks via a read-only
REST endpoint at GET /api/v1/@apostrophecms/global/nav. This allows
the Next.js marketing app to consume navigation data without
authentication. Relationship fields (_page) are automatically resolved
by Apostrophe's middleware before the handler runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: SashaMarchuk

website/modules/@apostrophecms/global/index.js

@@ -133,4 +133,18 @@ module.exports = {
       },
     },
   },
+  apiRoutes(self) {
+    return {
+      get: {
+        nav(req) {
+          const { global } = req.data;
+          return {
+            primaryNav: global.primaryNav || [],
+            footerLinks: global.footerLinks || [],
+            socialMediaLinks: global.socialMediaLinks || [],
+          };
+        },
+      },
+    };
+  },
 };