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

routes/index.js

@@ -14,6 +14,8 @@ const footerController = require("../controllers/footerController");
 const contactController = require("../controllers/contactController");
 const studentSupportController = require("../controllers/studentSupportController");
 const requestInfoController = require("../controllers/requestInfoController");
+const submissionController = require("../controllers/submissionController");
+const newsletterSubscriptionController = require("../controllers/newsletterSubscriptionController");
 
 const headerMenuController = require("../controllers/headerMenuController");
 const programmeController = require("../controllers/programmeController");
@@ -64,6 +66,8 @@ router.get("/api/request-info", requestInfoController.api);
 
 // Contact form submission (public)
 router.post("/api/contact/submit", contactController.submitForm);
+router.post("/api/submissions", submissionController.create);
+router.post("/api/newsletter/subscribe", newsletterSubscriptionController.subscribe);
 
 // Blog API Routes
 router.get("/api/blog", blogController.api);