[mailserver] Drop unneeded NET_ADMIN capability [ci skip]

## Context

The mailserver container had `capabilities.add = ["NET_ADMIN"]`. Upstream
docker-mailserver docs say the capability is only needed by Fail2ban to
run iptables ban actions. Fail2ban is DISABLED in this stack
(`ENABLE_FAIL2BAN=0`, see line ~68) — CrowdSec owns the brute-force
policy at the LB layer. The capability was therefore unused ballast and
a minor attack-surface reduction opportunity. Addresses code-4mu.

## This change

Replaces the explicit `capabilities { add = ["NET_ADMIN"] }` block with
an empty `security_context {}`. Post-rollout verification
(`supervisorctl status`) confirms every service we actually run is
healthy — dovecot, postfix, rspamd, rsyslog, postsrsd, changedetector,
cron, mailserver. Every STOPPED entry was already disabled.

The inline comment documents the revert trigger: check
`kubectl logs -c docker-mailserver` for permission-denied patterns and
restore the capability if observed.

## Test Plan

### Automated
```
$ kubectl get pod -n mailserver -l app=mailserver -o jsonpath='{.items[0].spec.containers[?(@.name=="docker-mailserver")].securityContext}'
{"allowPrivilegeEscalation":true,"privileged":false,"readOnlyRootFilesystem":false,"runAsNonRoot":false}

$ kubectl rollout status deployment/mailserver -n mailserver
deployment "mailserver" successfully rolled out

$ kubectl exec -n mailserver -c docker-mailserver deployment/mailserver -- \
    supervisorctl status | grep RUNNING
changedetector RUNNING ...
cron           RUNNING ...
dovecot        RUNNING ...
mailserver     RUNNING ...
postfix        RUNNING ...
postsrsd       RUNNING ...
rspamd         RUNNING ...
rsyslog        RUNNING ...
```

### Observation window
EmailRoundtripFailing + EmailRoundtripStale alerts continue to run
every 20 min. If no alert fires in the 24h post-rollout window
(through ~2026-04-20 10:40 UTC), the change is considered safe and
this commit stands. Otherwise revert this commit.

## What is NOT in this change

- readOnlyRootFilesystem (separate hardening, out of scope)
- runAsNonRoot (docker-mailserver needs root for Postfix)
- Removing privilege-escalation defaults (container needs those for
  chowning mail spool at startup)

Closes: code-4mu

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stacks/mailserver/modules/mailserver/main.tf

@@ -264,11 +264,14 @@ resource "kubernetes_deployment" "mailserver" {
           name              = "docker-mailserver"
           image             = "docker.io/mailserver/docker-mailserver:15.0.0"
           image_pull_policy = "IfNotPresent"
-          security_context {
-            capabilities {
-              add = ["NET_ADMIN"]
-            }
-          }
+          # NET_ADMIN was originally required by docker-mailserver's
+          # Fail2ban (iptables ban actions). Fail2ban is DISABLED in this
+          # stack (ENABLE_FAIL2BAN=0, see above) — CrowdSec owns the
+          # brute-force policy. The capability is therefore unnecessary.
+          # Dropping it 2026-04-19 (code-4mu). If mail flow regresses,
+          # `kubectl logs -n mailserver -l app=mailserver -c docker-mailserver`
+          # will show permission-denied errors — revert if observed.
+          security_context {}
 
           lifecycle {
             post_start {