Fix bugs found in second multi-agent review

XSS / security:
- markdown.js: sanitize footnote labels to a safe slug before using them in
  id/href attributes. Labels are captured before the HTML-escape pass, so a
  label like x"><img onerror=...> broke out → stored XSS (the earlier quote-
  escape fix didn't cover this path). Verified neutralized.
- RateLimitMiddleware: only trust X-Forwarded-For / X-Real-IP when REMOTE_ADDR
  is a configured trusted proxy, and use the rightmost (proxy-appended) entry.
  Previously any client could rotate XFF to escape the per-IP rate limit.
- .env.example: document TRUSTED_PROXIES so fresh deploys aren't fail-open on
  the Authelia forward-auth spoofing protection.

Correctness:
- notifications.php: my previous assigned-to LIKE fix anchored only on '}', so
  BULK assignments (logged {"assigned_to":N,"bulk_operation_id":..}) produced
  no "assigned to you" notification — now matches both '}' and ',' delimiters.
- notifications.php: implement the documented @mention notifications (query
  action_type='mention' rows for the current user); they were never delivered.
- NotificationHelper::notifyWatchers: guard unchecked prepare() so a missing
  ticket_watchers table can't fatal the request after its DB write committed.
- AuditLogModel::getTicketTimeline: JSON_UNQUOTE the extracted ticket_id so
  comment events actually match (string vs JSON-number comparison never did).
- AuditLogModel/audit_log.php: CSV export no longer silently truncates to the
  1000-row UI cap; uses a dedicated higher export limit.
- DashboardView: quick-preview drawer read .ticket-link from the title cell
  (which has none), so the title was always blank — use the cell text.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api/audit_log.php

@@ -46,8 +46,10 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
             $filters['ip_address'] = $_GET['ip_address'];
         }
 
-        // Get all matching logs (no limit for CSV export)
-        $result = $auditLogModel->getFilteredLogs($filters, 10000, 0);
+        // Get all matching logs for export. The forExport flag raises the cap
+        // (model clamps to its export limit) so the CSV isn't silently truncated
+        // to the 1000-row UI page limit.
+        $result = $auditLogModel->getFilteredLogs($filters, PHP_INT_MAX, 0, true);
         $logs = $result['logs'];
 
         // Set CSV headers