feat(mail): tenant alias and distribution-list management via Stalwart

Customer-admin Mail settings backed by Stalwart JMAP: per-tenant aliases
(extra addresses routing to a mailbox) and distribution lists (one address
fanning out to many recipients). Adds StalwartClient x:Alias/x:MailingList
methods, a tenant-scoped MailController/MailService, the portal Mail settings
page and its proxy routes, and the mailboxAddress field on TenantUserDoc.
Removes the old mock mail data now that the page reads live data.

apps/portal/server/api/tenants/[slug]/mail/aliases.post.ts

@@ -0,0 +1,16 @@
+// Create an email alias. Proxies POST /tenants/:slug/mail/aliases.
+import { getUserSession } from 'nuxt-oidc-auth/runtime/server/utils/session.js'
+
+export default defineEventHandler(async (event) => {
+  const session = await getUserSession(event).catch(() => null)
+  const accessToken = (session as { accessToken?: string } | null)?.accessToken
+  if (!accessToken) throw createError({ statusCode: 401, statusMessage: 'Not signed in' })
+  const slug = getRouterParam(event, 'slug')
+  const body = await readBody(event)
+  const base = process.env.PLATFORM_API_INTERNAL_URL ?? 'http://platform-api:3001'
+  return $fetch(`${base}/tenants/${slug}/mail/aliases`, {
+    method: 'POST',
+    headers: { Authorization: `Bearer ${accessToken}` },
+    body,
+  })
+})