feat: ticket watchers, fulltext search, single-query pagination, watcher notifications

Ticket watchers:
- api/watch_ticket.php: GET (watch state) + POST (watch/unwatch toggle)
- index.php: route for /api/watch_ticket.php
- TicketView: WATCH/UNWATCH button with live state fetch and toggle
- NotificationHelper::notifyWatchers(): fetches watchers from DB, resolves
  Matrix IDs via Synapse, fires notification to watchers + global list
- add_comment.php, update_ticket.php: call notifyWatchers on comment and
  status-change events respectively

Fulltext search:
- TicketModel::hasFulltextIndex(): detects FULLTEXT index via information_schema
- getAllTickets(): uses MATCH...AGAINST when fulltext index exists, LIKE fallback
  when not yet applied — zero-downtime rollout

Single-query pagination:
- getAllTickets() replaces separate COUNT + SELECT with COUNT(*) OVER() window
  function — one round trip to DB per page load instead of two

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

api/add_comment.php

@@ -144,6 +144,13 @@ try {
             NotificationHelper::sendCommentNotification($ticketId, $ticketTitle, $commentText, $authorDisplay);
         }
 
+        // Notify watchers of the new comment
+        NotificationHelper::notifyWatchers(
+            $conn, $ticketId, $ticketTitle, 'comment_added',
+            ['author' => $authorDisplay, 'preview' => mb_strimwidth($commentText, 0, 200, '…')],
+            (int)$userId
+        );
+
         // Add mentioned users to result for frontend
         $result['mentions'] = array_map(function($u) {
             return $u['username'];