feat(global): add public API endpoint for navigation data (#266)

## Summary
- Adds a read-only REST endpoint at `GET
/api/v1/@apostrophecms/global/nav`
- Returns `{ primaryNav, footerLinks, socialMediaLinks }` with resolved
`_page` relationships
- No authentication required — enables the Next.js marketing app to
consume nav data
- Zero new dependencies, zero database changes, zero impact on existing
behavior

## Details
Uses Apostrophe's built-in `apiRoutes` pattern (same as the form
module's `routes`). The handler reads from `req.data.global`, which is
already populated by Apostrophe's global middleware on every request —
no extra DB queries.

**1 file changed, 14 lines added:**
`website/modules/@apostrophecms/global/index.js`

## Test plan
- [x] ESLint passes
- [x] All 196 tests pass (`npm run test`)
- [x] Runtime verified via Docker — endpoint returns clean JSON
- [ ] `curl http://localhost:3000/api/v1/@apostrophecms/global/nav`
returns nav JSON
- [ ] Existing CMS admin pages unaffected
- [ ] No auth token needed for the endpoint

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
Adds a public GET /api/v1/@apostrophecms/global/nav endpoint exposing
navigation data (primaryNav, footerLinks, socialMediaLinks) as JSON.
Implemented via Apostrophe’s apiRoutes, handler reads req.data.global
(defensive guard defaults to {}), returns empty-array fallbacks,
requires no authentication, adds no deps or schema changes, and avoids
extra database queries or runtime changes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Sasha.Marchuk <Sasha.Marchuk.email@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

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

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