FEAT(thanos): add Thanos for Prometheus HA and long-term storage

- Add Thanos Query, Store Gateway, Compactor
- Enable Prometheus Sidecar with S3 (MinIO) storage
- Configure Prometheus replicas: 2 with pod anti-affinity
- Add ExternalSecrets for MinIO credentials
- Retention: raw 7d, 5m downsampled 30d, 1h downsampled 90d

kustomization.yaml

@@ -4,6 +4,7 @@ kind: Kustomization
 resources:
   - application.yaml
   - prometheus/argocd.yaml
+  - thanos/argocd.yaml
   - alertmanager/argocd.yaml
   - grafana/argocd.yaml
   - loki/argocd.yaml