feat(admin): implement unified submission and newsletter management

Introduce a centralized system to manage all website form submissions and newsletter subscriptions.
- Add `Submission` and `NewsletterSubscription` models with MongoDB schema validation
- Implement `submissionController` and `newsletterSubscriptionController` for CRUD operations and filtering
- Create a unified admin UI for reviewing submissions across different sources (home, request, contact, partnership, newsletter)
- Add database migration scripts for creating collections and indexes
- Refactor partnership inquiry forms to use a fixed field structure
- Update admin navigation and server CORS settings to support PATCH requests

server.js

@@ -144,7 +144,7 @@ app.use((req, res, next) => {
     
     if (isOriginAllowed) {
         res.setHeader("Access-Control-Allow-Origin", origin || "*");
-        res.setHeader("Access-Control-Allow-Methods", "GET,POST,PUT,DELETE,OPTIONS");
+        res.setHeader("Access-Control-Allow-Methods", "GET,POST,PUT,PATCH,DELETE,OPTIONS");
         res.setHeader("Access-Control-Allow-Headers", "Content-Type, Authorization");
         res.setHeader("Access-Control-Allow-Credentials", "true");
     }