fix(gateway): add /api/app/version route for mobile update check

Mobile app was calling /api/app/version/check but this route was not
configured in Kong gateway. Added admin-mobile-version route to forward
these requests to admin-service.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/api-gateway/kong.yml

@@ -188,6 +188,10 @@ services:
         paths:
           - /api/v1/admin
         strip_path: false
+      - name: admin-mobile-version
+        paths:
+          - /api/app/version
+        strip_path: false
 
   # ---------------------------------------------------------------------------
   # Presence Service - 在线状态服务