fix: NFS outage recovery — migrate to NFSv4, add alerting

NFS server restart broke NFSv3 (lockd kernel bug on PVE 6.14).
All 52 NFS PVs patched to nfsvers=4, NFSv3 disabled on PVE.

Changes:
- nfs_volume module: add nfsvers=4 mount option
- nfs-csi StorageClass: add nfsvers=4 mount option
- dbaas: MySQL serverInstances 3→1, mysql-native-password=ON
- monitoring: add NFSCSINodeDown and NFSMountFailures alerts

[ci skip]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stacks/dbaas/modules/dbaas/main.tf

@@ -180,7 +180,7 @@ resource "helm_release" "mysql_cluster" {
   version    = "2.2.7"
 
   values = [yamlencode({
-    serverInstances = 3
+    serverInstances = 1
     routerInstances = 1
     serverVersion   = "8.4.4"
 
@@ -216,6 +216,7 @@ resource "helm_release" "mysql_cluster" {
       mycnf = <<-EOT
         [mysqld]
         skip-name-resolve
+        mysql-native-password=ON
         # Auto-recovery after crashes: rejoin group without manual intervention
         group_replication_autorejoin_tries=2016
         group_replication_exit_state_action=OFFLINE_MODE