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/assign_ticket.php

@@ -3,6 +3,8 @@ require_once __DIR__ . '/bootstrap.php';
 require_once dirname(__DIR__) . '/models/TicketModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
 require_once dirname(__DIR__) . '/models/UserModel.php';
+require_once dirname(__DIR__) . '/helpers/NotificationHelper.php';
+require_once dirname(__DIR__) . '/helpers/SynapseHelper.php';
 
 // Get request data
 $data = json_decode(file_get_contents('php://input'), true);
@@ -60,6 +62,21 @@ if ($assignedTo === null || $assignedTo === '') {
     $success = $ticketModel->assignTicket($ticketId, $assignedTo, $userId);
     if ($success) {
         $auditLogModel->log($userId, 'assign', 'ticket', $ticketId, ['assigned_to' => $assignedTo]);
+
+        if (!empty($GLOBALS['config']['MATRIX_NOTIFY_ASSIGNMENTS'])) {
+            $changedByDisplay = $currentUser['display_name'] ?? $currentUser['username'] ?? null;
+            $assigneeName     = $targetUser['display_name'] ?? $targetUser['username'] ?? null;
+            $assigneeMatrix   = isset($targetUser['username'])
+                                ? SynapseHelper::resolveUsername($targetUser['username'])
+                                : null;
+            NotificationHelper::sendAssignmentNotification(
+                $ticketId,
+                $ticket['title'] ?? "Ticket #{$ticketId}",
+                $assigneeName,
+                $assigneeMatrix,
+                $changedByDisplay
+            );
+        }
     }
 }