resilience improvements: MySQL anti-affinity comment, descheduler 5min, prometheus termination 60s

- MySQL InnoDB: keep required anti-affinity but document why (2/3 members OK during node loss)
- Descheduler: increase frequency from hourly to every 5 min for faster rebalancing
- Prometheus: set terminationGracePeriodSeconds=60 to prevent drain timeout [ci skip]

stacks/dbaas/modules/dbaas/main.tf

@@ -237,6 +237,10 @@ resource "helm_release" "mysql_cluster" {
           }
         }
         podAntiAffinity = {
+          # Required anti-affinity: MySQL pods MUST be on different nodes.
+          # During node loss, one pod will be Pending — this is acceptable because
+          # InnoDB Cluster operates with 2/3 members (OK_NO_TOLERANCE).
+          # The descheduler (every 5 min) handles violations if any occur.
           requiredDuringSchedulingIgnoredDuringExecution = [{
             labelSelector = {
               matchLabels = {