feat(scheduling): dezky Scheduling — Calendly-style booking on Stalwart calendars

First-party booking system on top of Stalwart calendars (no third-party
scheduling dependency). Hosts expose public booking pages; visitors pick a
slot computed from the host's live Stalwart free/busy, and confirming writes
the event to the host's calendar and sends a dezky-branded confirmation with
an .ics.

platform-api (services/platform-api/src/scheduling):
- Schemas: Host, StalwartCredential (AES-256-GCM at rest), AvailabilitySchedule,
  EventType, Booking, SlotLock (unique (hostId,startUtc) + TTL).
- StalwartCalendarModule: JMAP gateway (free/busy via Principal/getAvailability,
  event create/delete, scheduleAgent=client) + on-behalf app-password
  provisioning. CredentialCipher for at-rest encryption.
- DST-correct slot engine (Luxon) with unit tests; two-layer double-booking
  guard (atomic SlotLock + live free/busy re-check).
- Booking confirm/cancel/reschedule, branded email + .ics via JMAP submission,
  self-service manage tokens. /api/v1 public + tenant-gated admin routes,
  per-IP rate limiting.

apps/booking: standalone public, whitelabel booking app (booking.dezky.eu) —
path-based tenant resolution, per-tenant brand colour, booking + manage flows.

apps/portal: admin scheduling page (hosts, event types, availability, bookings
with edit/delete + admin cancel/reschedule) and proxy routes.

infra: booking dev service in docker-compose; scheduling env vars.

apps/portal/components/PortalSidebar.vue

@@ -62,6 +62,7 @@ const ADMIN_NAV: NavRow[] = [
   { id: 'users',        label: 'Users & groups',   icon: 'users',    href: '/admin/users' },
   { id: 'mail',         label: 'Mail settings',    icon: 'mail',     href: '/admin/mail' },
   { id: 'meetings',     label: 'Meetings',         icon: 'video',    href: '/admin/meetings' },
+  { id: 'scheduling',   label: 'Scheduling',       icon: 'calendar', href: '/admin/scheduling' },
   { id: 'chat',         label: 'Chat',             icon: 'chat',     href: '/admin/chat' },
   { id: 'domains',      label: 'Domains',          icon: 'globe',    href: '/admin/domains' },
   { id: 'storage',      label: 'Storage',          icon: 'database', href: '/admin/storage' },
@@ -121,6 +122,7 @@ const currentId = computed(() => {
   if (p.startsWith('/admin/users')) return 'users'
   if (p.startsWith('/admin/mail')) return 'mail'
   if (p.startsWith('/admin/meetings')) return 'meetings'
+  if (p.startsWith('/admin/scheduling')) return 'scheduling'
   if (p.startsWith('/admin/chat')) return 'chat'
   if (p.startsWith('/admin/domains')) return 'domains'
   if (p.startsWith('/admin/storage')) return 'storage'