fix: eliminate memory overcommit to prevent node OOM crashes

Set requests = limits (Guaranteed QoS) across LimitRange defaults and
explicit pod resources. Node2 crashed 2026-03-14 from 250% memory
overcommit (61GB limits on 24GB node).

Changes:
- LimitRange: default = defaultRequest for all 6 tiers
- Grafana: 3 → 2 replicas
- Grampsweb: document why replicas=0
- Prometheus: 1Gi/4Gi → 3Gi/3Gi
- OpenClaw: 512Mi/2Gi → 768Mi/768Mi
- Immich server: 256Mi/2Gi → 512Mi/512Mi
- Immich postgresql: 256Mi/1Gi → 512Mi/512Mi
- Calibre: 256Mi/1536Mi → 256Mi/256Mi
- Linkwarden: 256Mi/1536Mi → 768Mi/768Mi
- N8N: 256Mi/1Gi → 512Mi/512Mi
- MySQL cluster: 1Gi/3-4Gi → 2Gi/2Gi
- pg-cluster (CNPG): 512Mi/4Gi → 512Mi/512Mi
- DBaaS ResourceQuota limits.memory: 64Gi → 12Gi

[ci skip]

stacks/immich/main.tf

@@ -246,10 +246,10 @@ resource "kubernetes_deployment" "immich_server" {
           resources {
             requests = {
               cpu    = "100m"
-              memory = "256Mi"
+              memory = "512Mi"
             }
             limits = {
-              memory = "2Gi"
+              memory = "512Mi"
             }
           }
         }
@@ -378,10 +378,10 @@ resource "kubernetes_deployment" "immich-postgres" {
           resources {
             requests = {
               cpu    = "50m"
-              memory = "256Mi"
+              memory = "512Mi"
             }
             limits = {
-              memory = "1Gi"
+              memory = "512Mi"
             }
           }
         }