Add assignment dropdown on ticket creation and fix Discord webhook URLs

- Add APP_DOMAIN config for correct Discord webhook ticket links
- Add "Assign To" dropdown on create ticket form
- Update TicketModel.createTicket() to support assigned_to field
- Update documentation for APP_DOMAIN requirement

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/js/dashboard.js

@@ -643,6 +643,10 @@ function loadTemplate() {
         document.getElementById('priority').value = '4';
         document.getElementById('category').value = 'General';
         document.getElementById('type').value = 'Issue';
+        const assignedToSelect = document.getElementById('assigned_to');
+        if (assignedToSelect) {
+            assignedToSelect.value = '';
+        }
         return;
     }