feat(ansible): standardize Neo4j ports and add monitoring

- Unify Neo4j HTTP/Bolt/syslog ports across ariel and umbriel hosts
- Add neo4j_metrics_port (22094) for APOC exporter sidecar
- Add umbriel to Prometheus node_exporter targets
- Add Neo4j scrape config and alerts for tx rollback rate and
  stalled store growth
- Replace kernos_harper MCP with andromeda (caliban.helu.ca)
- Remove angelia MCP from kottos fastagent config
- Switch neo4j group membership from keeper_user to ponos

ansible/pplg/alert_rules.yml.j2

@@ -384,6 +384,48 @@ groups:
           summary: "Mnemosyne Celery backlog on {{ $labels.queue }}"
           description: "Celery queue '{{ $labels.queue }}' has {{ $value }} pending tasks for more than 10 minutes — check the worker logs in Loki ({service=\"mnemosyne\", component=\"worker\"})."
 
+  # ============================================================================
+  # Neo4j Alerts (neo4j-apoc-exporter sidecar)
+  # ============================================================================
+  # Metrics come from stscoundrel/neo4j-apoc-exporter, which connects to
+  # Neo4j over Bolt and surfaces apoc.monitor.* gauges plus standard JVM
+  # metrics.  "Exporter down" therefore covers both "exporter container
+  # crashed" and "exporter cannot reach Bolt" — either way Neo4j is
+  # effectively unobservable.  Hostname-only — purpose of each instance
+  # is implied by the host (e.g. ariel = LLM memory, umbriel = Mnemosyne).
+  - name: neo4j_alerts
+    rules:
+      - alert: Neo4jExporterDown
+        expr: up{job="neo4j"} == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Neo4j exporter down on {{ $labels.instance }}"
+          description: "The neo4j-apoc-exporter on {{ $labels.instance }} has been unreachable for more than 5 minutes.  Either the sidecar container is down or it cannot connect to Neo4j over Bolt — check `docker ps` and `docker logs neo4j-exporter` on the host."
+
+      - alert: Neo4jHighRollbackRate
+        expr: |
+          rate(neo4j_monitor_tx_rolledBackTx[10m])
+            / clamp_min(rate(neo4j_monitor_tx_totalOpenedTx[10m]), 1) > 0.10
+        for: 10m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Neo4j transaction rollback rate above 10% on {{ $labels.instance }}"
+          description: "More than 10% of transactions on {{ $labels.instance }} have rolled back over the last 10 minutes — check application logs in Loki ({job=\"neo4j\", hostname=\"{{ $labels.instance }}\"})."
+
+      - alert: Neo4jStoreGrowthStalled
+        expr: |
+          rate(neo4j_monitor_tx_totalOpenedTx[15m]) == 0
+            and neo4j_monitor_tx_currentOpenedTx > 0
+        for: 15m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Neo4j has open transactions but zero throughput on {{ $labels.instance }}"
+          description: "{{ $labels.instance }} shows {{ $value }} currently-open transactions but no new transactions opened in 15 minutes — possible Bolt-side hang or stuck query."
+
 # Red Panda Seal of Approval 🐼
 # "If the metrics aren't red, go back to bed"
 {% endraw %}