fix(security): add SRI to jQuery CDN, escape chat ID, use server-side hostname

- Add integrity/crossorigin attributes to jQuery 3.6.0 CDN script
- Use | int filter on zammad_chat_id to prevent XSS
- Replace request.headers.get('host') with settings.external_hostname
  for canonical URL and Open Graph tags to prevent host-header injection

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/views/help.py

@@ -33,6 +33,7 @@ async def help_center(request: Request):
         "help.html",
         {
             "request": request,
+            "external_hostname": settings.external_hostname,
             "zammad_url": settings.zammad_url,
             "zammad_chat_enabled": settings.zammad_chat_enabled,
             "zammad_chat_id": settings.zammad_chat_id,