feat: comment pagination, Matrix integration, Synapse mention resolution

Comment pagination:
- CommentModel: add getCommentCount(), paginated getCommentsByTicketId()
  with getThreadedCommentsPaged() for threading + LIMIT/OFFSET
- TicketController: load first 50 root comments + total count on page load
- api/get_comments.php: new AJAX endpoint for Load More (index.php routed)
- TicketView: Load More button + buildCommentEl() JS renderer for AJAX comments;
  passes totalComments/commentOffset/isAdmin to window.ticketData

Matrix integration:
- NotificationHelper: add sendStatusChangeNotification(), sendCommentNotification(),
  sendMentionNotification(), sendAssignmentNotification() alongside existing
  sendTicketNotification(); internal fire() helper replaces duplicated cURL logic
- SynapseHelper: new helper that resolves SSO usernames → Matrix IDs by querying
  Synapse Admin REST API directly (no caching, no stale data)
- config.php: add SYNAPSE_ADMIN_URL, SYNAPSE_ADMIN_TOKEN, MATRIX_NOTIFY_COMMENTS,
  MATRIX_NOTIFY_ASSIGNMENTS config keys (all from .env)
- api/update_ticket.php: fire status-change notification after successful save
- api/add_comment.php: resolve @mentioned usernames via SynapseHelper and fire
  mention notification; fire general comment notification when MATRIX_NOTIFY_COMMENTS=1
- api/assign_ticket.php: fire assignment notification (resolves assignee via Synapse)
  when MATRIX_NOTIFY_ASSIGNMENTS=1

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

api/update_ticket.php

@@ -26,6 +26,7 @@ try {
     require_once $commentModelPath;
     require_once $auditLogModelPath;
     require_once $workflowModelPath;
+    require_once dirname(__DIR__) . '/helpers/NotificationHelper.php';
 
     // Check authentication via session
     if (session_status() === PHP_SESSION_NONE) {
@@ -207,6 +208,18 @@ try {
                 }
             }
 
+            // Notify on status change
+            if ($currentTicket['status'] !== $updateData['status']) {
+                $changedBy = $this->currentUser['display_name'] ?? $this->currentUser['username'] ?? null;
+                NotificationHelper::sendStatusChangeNotification(
+                    $id,
+                    $currentTicket['status'],
+                    $updateData['status'],
+                    $updateData['title'],
+                    $changedBy
+                );
+            }
+
             return [
                 'success'    => true,
                 'status'     => $updateData['status'],