fix(infra): Stalwart v0.16 management admin is a real account (admin@dezky.eu)

The v0.16 config migration silently dropped the fallback admin — the live
server had ZERO accounts, so every platform-api JMAP call 401'd and tenant
mail provisioning was dead. Bootstrapped via recovery mode on node1
(STALWART_RECOVERY_ADMIN): created the dezky.eu domain + an admin account
with the Admin role and the existing STALWART_ADMIN_PASSWORD.

v0.16 logins use the full address, so STALWART_ADMIN_USER becomes
admin@dezky.eu; config-rev annotation bump rolls platform-api so it picks
up the new env. install.sh follow-ups now document the recovery-mode
bootstrap for rebuilds instead of the defunct fallback-admin promise.

infrastructure/production/fleet/apps/platform-api-config.yaml

@@ -12,7 +12,11 @@ data:
   # Stalwart runs on the HOST (not k3s). Pods reach it via the cni0 gateway IP
   # on the JMAP management port; the firewall lets the pod CIDR through.
   STALWART_API_URL: "http://10.42.0.1:8080"
-  STALWART_ADMIN_USER: "admin"
+  # Full address, not bare "admin": Stalwart v0.16 has no fallback admin —
+  # the management credential is a real account (admin@dezky.eu, Admin role)
+  # created during the 2026-06-10 recovery-mode bootstrap, and v0.16 logins
+  # use the full address.
+  STALWART_ADMIN_USER: "admin@dezky.eu"
   STALWART_PROVISIONING_ENABLED: "true"
   # Base for per-tenant service mail domains ({slug}.dezky.eu) AND the
   # reserved namespace for customer domains: only the dezky tenant may claim