feat: add monitoring resources (alerts, dashboard, service-monitor)

- PrometheusRule with 16 alerts across 6 groups:
  availability, ingest, query, dependencies, resources, errors
- Grafana dashboard (ConfigMap, sidecar auto-discovery):
  8 rows covering overview, ingest pipeline, query perf,
  context retrieval, database, relevance, error breakdown,
  app resources, logs
- ServiceMonitor moved from k8s/app/ to k8s/monitoring/
- Dashboard generator script (Python, matches homelab pattern)
- Separate k8s/monitoring/ path for ArgoCD Application
- sqlx::migrate!() added to server startup for seamless migrations

k8s/monitoring/service-monitor.yaml

@@ -0,0 +1,20 @@
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: poimen-memory
+  namespace: monitoring
+  labels:
+    release: kube-prometheus-stack
+    app.kubernetes.io/name: poimen-memory
+spec:
+  namespaceSelector:
+    matchNames:
+      - poimen
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: poimen-memory
+  endpoints:
+    - port: http
+      path: /metrics
+      interval: 30s
+      scrapeTimeout: 10s