[ci skip] expand Prometheus PVC to 200Gi, increase retention to 180GB for 1-year history

Storage analysis: ~10.5 GB/month ingestion rate, 1 year = ~125 GB + overhead.
PVC: 30Gi → 200Gi, retention.size: 45GB → 180GB.
Historical TSDB data restored from NFS (39.8 GB total including all blocks).

stacks/platform/modules/monitoring/prometheus.tf

@@ -11,7 +11,7 @@ resource "kubernetes_persistent_volume_claim" "prometheus_server_pvc" {
     storage_class_name = "iscsi-truenas"
     resources {
       requests = {
-        storage = "30Gi"
+        storage = "200Gi"
       }
     }
   }

stacks/platform/modules/monitoring/prometheus_chart_values.tpl

@@ -114,7 +114,7 @@ server:
     # - "web.enable-admin-api"
     - "web.enable-lifecycle"
     - "storage.tsdb.allow-overlapping-blocks"
-    - "storage.tsdb.retention.size=45GB"
+    - "storage.tsdb.retention.size=180GB"
     - "storage.tsdb.wal-compression"
   persistentVolume:
     # enabled: false