feat(observability): add SearXNG, Argos, and Pallas monitoring

- Add SearXNG syslog ingestion and blackbox health probes on miranda
  and rosalind for per-host attributable failure detection
- Scrape Argos MCP application metrics from miranda
- Add Pallas dashboard panels for downstream availability and turn
  error ratios

ansible/site.yml

@@ -33,6 +33,9 @@
 - name: Deploy SearXNG
   import_playbook: searxng/deploy.yml
 
+- name: Deploy SearXNG OAuth2-Proxy sidecar
+  import_playbook: searxng/deploy_oauth2.yml
+
 - name: Deploy HAProxy
   import_playbook: haproxy/deploy.yml