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/get_comments.php

@@ -0,0 +1,46 @@
+<?php
+/**
+ * Get Comments API
+ * Returns paginated comments for a ticket (used by "Load more" on ticket view)
+ */
+
+require_once __DIR__ . '/bootstrap.php';
+require_once dirname(__DIR__) . '/models/CommentModel.php';
+require_once dirname(__DIR__) . '/models/TicketModel.php';
+
+if ($_SERVER['REQUEST_METHOD'] !== 'GET') {
+    http_response_code(405);
+    echo json_encode(['success' => false, 'error' => 'Method not allowed']);
+    exit;
+}
+
+$ticketId = isset($_GET['ticket_id']) ? (int)$_GET['ticket_id'] : 0;
+$offset   = isset($_GET['offset'])    ? max(0, (int)$_GET['offset'])   : 0;
+$limit    = isset($_GET['limit'])     ? min(100, max(1, (int)$_GET['limit'])) : 50;
+
+if ($ticketId <= 0) {
+    http_response_code(400);
+    echo json_encode(['success' => false, 'error' => 'Invalid ticket_id']);
+    exit;
+}
+
+$ticketModel = new TicketModel($conn);
+$ticket = $ticketModel->getTicketById($ticketId);
+if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
+    http_response_code(404);
+    echo json_encode(['success' => false, 'error' => 'Ticket not found']);
+    exit;
+}
+
+$commentModel  = new CommentModel($conn);
+$total         = $commentModel->getCommentCount($ticketId);
+$comments      = $commentModel->getCommentsByTicketId($ticketId, true, $limit, $offset);
+
+echo json_encode([
+    'success'  => true,
+    'comments' => $comments,
+    'total'    => $total,
+    'offset'   => $offset,
+    'limit'    => $limit,
+    'has_more' => ($offset + $limit) < $total,
+]);