fix: restore pvc-autoresizer by allow-listing kubelet_volume_stats_available_bytes

The Prometheus scrape config for the kubernetes-nodes job kept
capacity_bytes + used_bytes but dropped available_bytes. pvc-autoresizer
computes utilization from available/capacity, so without that metric it
was silent for every PVC in the cluster — including mailserver, which
filled to 89% (1.7G/2.0G) and started rejecting all inbound mail with
'452 4.3.1 Insufficient system storage' (15+ hours, all real senders:
Brevo, Gmail, Facebook).

Also bumps the floors of mailserver (2Gi -> 5Gi, limit 10Gi) and forgejo
(15Gi -> 30Gi) PVCs to recover from the immediate outage, and adds
ignore_changes on requests.storage so future autoresizer expansions
don't cause TF drift.

stacks/forgejo/main.tf

@@ -40,10 +40,16 @@ resource "kubernetes_persistent_volume_claim" "data_encrypted" {
     storage_class_name = "proxmox-lvm-encrypted"
     resources {
       requests = {
-        storage = "15Gi"
+        storage = "30Gi"
       }
     }
   }
+  lifecycle {
+    # pvc-autoresizer expands this PVC up to storage_limit; ignore drift on
+    # requests.storage. To bump the floor manually: temporarily remove this
+    # block, apply the new size, re-add the block, apply again.
+    ignore_changes = [spec[0].resources[0].requests]
+  }
 }
 
 resource "kubernetes_deployment" "forgejo" {