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>

controllers/TicketController.php

@@ -49,8 +49,10 @@ class TicketController {
             return;
         }
 
-        // Get comments for this ticket using CommentModel
-        $comments = $this->commentModel->getCommentsByTicketId($id);
+        // Load first page of comments; show "load more" if ticket has many
+        $commentPageSize  = 50;
+        $totalComments    = $this->commentModel->getCommentCount((int)$id);
+        $comments         = $this->commentModel->getCommentsByTicketId($id, true, $commentPageSize, 0);
 
         // Get timeline for this ticket
         $timeline = $this->auditLogModel->getTicketTimeline($id);