feat(platform): real email domains, mailboxes & member lifecycle

Wire the mail/identity stack to real Stalwart/Authentik/OCIS provisioning,
replacing the mocked Domains and Users pages.

Domains (customer-admin):
- StalwartClient: real JMAP management (v0.16 dropped REST) — create/list/delete
  email domains via x:Domain at the internal http://stalwart:8080 listener;
  DKIM auto-generated; the records to publish are read from the domain's
  dnsZoneFile. Gated by STALWART_PROVISIONING_ENABLED.
- New Domain collection + DomainsModule: add/list/recheck/set-DMARC/remove,
  tenant-membership-gated and audited.
- DnsVerifierService: verifies MX/SPF/DKIM/DMARC/ownership against a public
  resolver (1.1.1.1/8.8.8.8) and diffs them against the expected records.
- Remove is guarded: refuses while accounts/aliases/mailing lists still use the
  domain (via Stalwart referential integrity).
- Domains page + add wizard on real data; sidebar badge counts domains needing
  attention.

Users & groups (customer-admin):
- Create a member provisioned across Authentik SSO, a Stalwart mailbox on the
  tenant's primary domain, and OCIS — returning a one-time password.
- Lifecycle: suspend/resume (Authentik is_active + freeze the mailbox via
  account permissions, original password preserved), force-logout (terminate
  sessions, filtered client-side so it can never end other users' sessions),
  reset password (new one-time password on SSO + mailbox), and remove (tear down
  mailbox + SSO identity + OCIS + doc; mailbox-in-use aware for multi-tenant
  users). Self-suspend / self-force-logout are blocked.

Infra: point platform-api at the internal Stalwart listener; document the new
STALWART_/provisioning vars in .env.example.

.env.example

@@ -46,7 +46,16 @@ OPERATOR_OIDC_CLIENT_SECRET=changeme_run_openssl_rand_hex_64
 # ────────────────────────────────────────
 # Stalwart Mail
 # ────────────────────────────────────────
+# Fallback admin login (config.toml authentication.fallback-admin). platform-api
+# uses admin + this password for Basic auth on the JMAP management API.
+STALWART_ADMIN_USER=admin
 STALWART_ADMIN_PASSWORD=changeme_use_openssl_rand
+# HMAC secret Stalwart signs its audit webhook POSTs with (verified by
+# platform-api at /ingest/stalwart/webhook). openssl rand -hex 32
+STALWART_WEBHOOK_SECRET=changeme_use_openssl_rand_hex_32
+# Set true to let platform-api create/delete domains + DKIM in Stalwart from the
+# customer-admin Domains page. Off by default (domain steps record 'skipped').
+STALWART_PROVISIONING_ENABLED=false
 
 # ────────────────────────────────────────
 # OCIS