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>

views/DashboardView.php

@@ -1342,7 +1342,7 @@ if (advForm) advForm.addEventListener('submit', function(e) {
     var o = hasCheckbox ? 1 : 0; // column offset for checkbox col
 
     var priority  = cells[1 + o] ? cells[1 + o].textContent.trim() : '';
-    var title     = cells[2 + o] ? cells[2 + o].querySelector('.ticket-link')?.textContent.trim() || '' : '';
+    var title     = cells[2 + o] ? cells[2 + o].textContent.trim() : '';
     var category  = cells[3 + o] ? cells[3 + o].textContent.trim() : '';
     var typeVal   = cells[4 + o] ? cells[4 + o].textContent.trim() : '';
     var status    = cells[5 + o] ? cells[5 + o].textContent.trim().replace(/^\s*●\s*/, '') : '';