reduce alert noise: raise memory thresholds, exclude claude-memory 4xx, right-size mysql-operator

- ContainerNearOOM: 85% → 90% (silences forgejo, changedetection, immich-pg, mysql-cluster)
- ClusterMemoryRequestsHigh: 85% → 92% (intentional overcommit)
- NodeMemoryPressureTrending: 85% → 92%
- HighService4xxRate: exclude claude-memory (401s from unauth requests are expected)
- mysql-operator memory limit: 512Mi → 580Mi (VPA upperBound 481Mi × 1.2)

stacks/dbaas/modules/dbaas/main.tf

@@ -81,7 +81,7 @@ resource "helm_release" "mysql_operator" {
         memory = "512Mi"
       }
       limits = {
-        memory = "512Mi"
+        memory = "580Mi"
       }
     }
   })]