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.

services/platform-api/package.json

@@ -9,7 +9,7 @@
     "start:dev": "nest start --watch",
     "start:debug": "nest start --debug --watch",
     "start:prod": "node dist/main",
-    "test": "jest",
+    "test": "NODE_OPTIONS=--experimental-vm-modules jest",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -19,9 +19,12 @@
     "@nestjs/platform-fastify": "^10.4.0",
     "@nestjs/config": "^3.3.0",
     "@nestjs/mongoose": "^10.1.0",
+    "@nestjs/throttler": "^6.2.1",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.1",
+    "ical-generator": "^8.0.1",
     "jose": "^5.9.0",
+    "luxon": "^3.5.0",
     "mongoose": "^8.7.0",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.0",
@@ -30,7 +33,11 @@
   "devDependencies": {
     "@nestjs/cli": "^10.4.0",
     "@nestjs/testing": "^10.4.0",
+    "@types/jest": "^29.5.12",
+    "@types/luxon": "^3.4.2",
     "@types/node": "^20.0.0",
+    "jest": "^29.7.0",
+    "ts-jest": "^29.2.5",
     "typescript": "^5.5.0",
     "ts-node": "^10.9.2"
   },