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/admin.js

@@ -19,6 +19,8 @@ const formController = require("../controllers/formController");
 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 pageController = require("../controllers/pageController");
 const settingController = require("../controllers/settingController");
@@ -199,6 +201,27 @@ router.put(
   contactController.updateSubmissionStatus,
 );
 
+// Unified submission management
+router.get("/submissions", ensureAuthenticated, submissionController.index);
+router.get("/submissions/data", ensureAuthenticated, submissionController.list);
+router.get("/submissions/:id", ensureAuthenticated, submissionController.detail);
+router.patch("/submissions/:id", ensureAuthenticated, submissionController.update);
+router.get(
+  "/newsletter-subscriptions/data",
+  ensureAuthenticated,
+  newsletterSubscriptionController.list,
+);
+router.get(
+  "/newsletter-subscriptions/:id",
+  ensureAuthenticated,
+  newsletterSubscriptionController.detail,
+);
+router.patch(
+  "/newsletter-subscriptions/:id",
+  ensureAuthenticated,
+  newsletterSubscriptionController.update,
+);
+
 // Student Support (LAMS static page)
 router.get(
   "/student-support",