feat(audit): add comprehensive audit logging with SIEM integration

- Add AuditLog model with append-only design (timestamp, user, action, resource, IP, details, severity)
- Add audit_service.py with record/query helpers and SIEM forwarding (Syslog RFC 5424, HTTP/webhook)
- Add /api/audit-logs REST endpoints with filtering and pagination
- Add /admin/audit-logs viewer UI with real-time filters
- Add SIEM config settings (syslog, HTTP for Splunk HEC/Logstash/Grafana Loki)
- Add Alembic migration 027_add_audit_logs
- Add navigation link in admin menu
- Add comprehensive tests (20 tests covering model, service, SIEM, API, view)

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

frontend/templates/base.html

@@ -171,6 +171,9 @@
                   <a href="/admin/backup" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-database w-4 mr-2 text-green-600" aria-hidden="true"></i> Backup &amp; Restore
                   </a>
+                  <a href="/admin/audit-logs" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                    <i class="fas fa-shield-halved w-4 mr-2 text-indigo-500" aria-hidden="true"></i> Audit Logs
+                  </a>
                   <a href="/status" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100"
                      {% if request and request.url.path == '/status' %}aria-current="page"{% endif %}>
                     <i class="fas fa-circle-dot w-4 mr-2 text-gray-500" aria-hidden="true"></i> Status