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/inventory/host_vars/prospero.incus.yml

@@ -74,6 +74,22 @@ prometheus_targets:
   - 'rosalind.incus:9100'
   - 'umbriel.incus:9100'
 
+# Neo4j scrape targets (neo4j-apoc-exporter sidecar on each Neo4j host)
+neo4j_metrics_targets:
+  - 'ariel.incus:22094'
+  - 'umbriel.incus:22094'
+
+# Pallas scrape targets — one entry per Pallas deployment (registry
+# port). The `instance` label distinguishes deployments; the `agent`
+# dimension comes from labels on the metrics themselves.
+pallas_metrics_targets:
+  - targets: ['caliban.incus:24000']
+    labels: {instance: iolaus}
+  - targets: ['caliban.incus:24100']
+    labels: {instance: kottos}
+  - targets: ['caliban.incus:24200']
+    labels: {instance: mentor}
+
 # Prometheus OAuth2-Proxy Sidecar
 prometheus_proxy_port: 9091
 prometheus_oauth2_proxy_dir: /etc/oauth2-proxy-prometheus
@@ -127,10 +143,23 @@ pgadmin_oauth_client_id: "{{ vault_pgadmin_oauth_client_id }}"
 pgadmin_oauth_client_secret: "{{ vault_pgadmin_oauth_client_secret }}"
 
 # ============================================================================
-# Casdoor Metrics (for Prometheus scraping)
+# Prometheus Metrics Scraping
 # ============================================================================
 
-casdoor_metrics_host: "titania.incus"
+# Casdoor
+casdoor_metrics_host: titania.incus
 casdoor_metrics_port: 22081
 casdoor_prometheus_access_key: "{{ vault_casdoor_prometheus_access_key }}"
 casdoor_prometheus_access_secret: "{{ vault_casdoor_prometheus_access_secret }}"
+
+# Daedalus Metrics
+daedalus_metrics_host: caliban.incus
+daedalus_metrics_port: 23081
+
+# Mnemosyne — two scrape targets:
+#   app: Django /metrics via nginx (django-prometheus + custom pipeline/MCP counters)
+#   web: nginx-prometheus-exporter sidecar (nginx stub_status → Prometheus format)
+mnemosyne_app_metrics_host: caliban.incus
+mnemosyne_app_metrics_port: 23181
+mnemosyne_web_metrics_host: caliban.incus
+mnemosyne_web_metrics_port: 23191