Fix loose comparisons, missing response codes, and session handling

- ticket.js: escape dependency_id with lt.escHtml() in data attribute
- assign_ticket.php: strict (int) cast for ticket_id (> 0 check), authorization
  comparisons, and add missing http_response_code(400) on invalid user ID
- TicketView.php: strict (int) cast for priority select, assigned_to select,
  and comment ownership check
- CommentModel.php: strict (int) cast for parent_comment_id thread comparison
- UserModel.php: strict (int) cast for is_admin check
- export_tickets.php: conditional session_start() to avoid double-start warning

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/js/ticket.js

@@ -512,7 +512,7 @@ function renderDependencies(dependencies) {
                         <span class="dependency-title">${lt.escHtml(dep.title)}</span>
                         <span class="status-badge ${statusClass}">${lt.escHtml(dep.status)}</span>
                     </div>
-                    <button data-action="remove-dependency" data-dependency-id="${dep.dependency_id}" class="lt-btn lt-btn-sm">REMOVE</button>
+                    <button data-action="remove-dependency" data-dependency-id="${lt.escHtml(String(dep.dependency_id))}" class="lt-btn lt-btn-sm">REMOVE</button>
                 </div>`;
             });