docs(architecture): fix stale 5-node claim -> 7 nodes (k8s-node1..6) [ci skip]

Cluster grew to 7 nodes (k8s-master + node1..6; node5/6 added ~10d ago)
but several docs still said "5 nodes". Corrected with live specs:

- overview.md: 7-node enumeration; node1 is 16c/48GB (doc wrongly said
  32GB), node2-6 are 8c/32GB general workers
- compute.md: "5-node" -> "7-node" cluster description
- dns.md: NodeLocal DNSCache DaemonSet "5 nodes" -> "7 nodes"
- mailserver.md: HAProxy backend diagram "node1..4" -> "node1..6"

Illustrative "0/5 nodes available" scheduler-error examples left as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/architecture/mailserver.md

@@ -22,7 +22,7 @@ flowchart TB
     MX --> PF[pfSense WAN<br/>vtnet0 192.168.1.2]
     PF -->|NAT rdr<br/>WAN:25/465/587/993<br/>→ 10.0.20.1:same| HAP
     HAP[pfSense HAProxy<br/>4 TCP frontends on 10.0.20.1<br/>send-proxy-v2 to backends]
-    HAP -->|round-robin<br/>tcp-check inter 120s| KN{k8s worker<br/>node1..4}
+    HAP -->|round-robin<br/>tcp-check inter 120s| KN{k8s worker<br/>node1..6}
     KN -->|NodePort 30125-30128<br/>ETP: Cluster → kube-proxy SNAT| PODEXT
 
     %% Internal ingress path