feat: prevent duplicate membership applications and fees

- Add UNIQUE constraint on membership_application.user_id (one app per user)
- Add UNIQUE constraint on membership_fees.user_id (one fee record per user)
- Add validation checks in process_application.php before inserting
- Improve error messages for duplicate submission attempts
- Add migration script to clean up existing duplicates before constraints
- Update checkMembershipApplication to set session message on redirect
- Add comprehensive documentation of duplicate prevention architecture

Individual payments/EFTs are tracked separately in payments table

src/config/functions.php

@@ -1434,6 +1434,10 @@ function checkMembershipApplication($user_id)
 
     // Check if the record exists and redirect
     if ($count > 0) {
+        // Set a session message before redirecting
+        if (!isset($_SESSION['message'])) {
+            $_SESSION['message'] = 'You have already submitted a membership application.';
+        }
         header("Location: membership_details.php");
         exit();
     }