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/booking/pages/index.vue

@@ -0,0 +1,23 @@
+<script setup lang="ts">
+// Root of booking.dezky.eu carries no tenant context — booking links are always
+// /:tenantSlug/:hostSlug/:eventTypeSlug. Show a neutral placeholder.
+useHead({ title: 'dezky Scheduling' })
+</script>
+
+<template>
+  <main class="wrap">
+    <div class="card">
+      <div class="brand">dezky · scheduling</div>
+      <h1>Booking</h1>
+      <p class="mute">Open the specific booking link you were given to choose a time.</p>
+    </div>
+  </main>
+</template>
+
+<style scoped>
+.wrap { min-height: 100vh; display: flex; align-items: center; justify-content: center; padding: 32px 16px; }
+.card { max-width: 460px; text-align: center; background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); box-shadow: var(--shadow); padding: 40px 28px; }
+.brand { font-size: 12px; letter-spacing: .08em; text-transform: uppercase; color: var(--text-mute); }
+h1 { font-size: 24px; margin: 8px 0; }
+.mute { color: var(--text-mute); }
+</style>