mysql: bump to 4Gi limit / 3Gi request; grow /srv/nfs LV to 3 TiB

mysql-standalone OOMKilled May 8 18:05 (anon-rss 2 GB at the 2 Gi limit).
innodb_buffer_pool_size=1Gi plus connection buffers and InnoDB internals
don't fit in 2 Gi. Bumping limit to 4 Gi (request 3 Gi) leaves headroom
without changing the buffer pool config.

/srv/nfs was at 90% (1.7T / 2T); grew the underlying pve/nfs-data LV
1 TiB online and ran resize2fs (now 60% used). Triggered by surfacing
during the 2026-05-09 IO-pressure post-mortem; thinpool had ~4.6 TiB
free.

The post-mortem also covers the stale-NFS-client trigger (legacy
/usr/local/bin/weekly-backup pointing at the decommissioned TrueNAS IP)
and the resulting wedged kthread on the PVE host. Script removed and
node_exporter restarted out-of-band; kthread will clear at next PVE
reboot. See docs/post-mortems/2026-05-09-io-pressure-stale-nfs.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

stacks/dbaas/modules/dbaas/main.tf

@@ -188,10 +188,10 @@ resource "kubernetes_stateful_set_v1" "mysql_standalone" {
           resources {
             requests = {
               cpu    = "250m"
-              memory = "1536Mi"
+              memory = "3Gi"
             }
             limits = {
-              memory = "2Gi"
+              memory = "4Gi"
             }
           }