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

views/admin/partnerships/partials/inquiry-form-tab.ejs

@@ -16,17 +16,13 @@
         <div class="mb-3">
           <div>
             <label class="form-label fw-semibold mb-1"><%= inquiryUi.fields?.label || "Form fields" %></label>
-            <div class="form-text mt-0"><%= inquiryUi.fieldsHelpText || inquiryUi.fields?.helpText || "" %></div>
+            <div class="form-text mt-0">
+              Field structure is fixed to match the inquiry form: First Name, Last Name, Organization Name, Partnership Type, and Message. You can edit field text, required state, width, and Partnership Type select options.
+            </div>
           </div>
         </div>
         <div id="inquiryFieldsList"></div>
-        <button type="button" class="cms-add-button mt-3" id="addInquiryFieldBtn">
-          <i class="fas fa-plus me-2"></i><%= inquiryUi.fields?.addLabel || "Add Field" %>
-        </button>
       </div>
     </div>
   </div>
 </div>
-
-
-