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/jest.config.mjs

@@ -0,0 +1,32 @@
+// Jest (ESM + TypeScript via ts-jest). The codebase is NodeNext ESM with `.js`
+// import specifiers, so we strip the extension in moduleNameMapper to resolve
+// `./x.js` → `./x.ts` under jest. Run with NODE_OPTIONS=--experimental-vm-modules
+// (set in the package.json "test" script).
+/** @type {import('ts-jest').JestConfigWithTsJest} */
+export default {
+  testEnvironment: 'node',
+  roots: ['<rootDir>/src'],
+  testMatch: ['**/?(*.)+(spec).ts'],
+  extensionsToTreatAsEsm: ['.ts'],
+  moduleNameMapper: {
+    '^(\\.{1,2}/.*)\\.js$': '$1',
+  },
+  transform: {
+    '^.+\\.ts$': [
+      'ts-jest',
+      {
+        useESM: true,
+        tsconfig: {
+          // Emit ESM regardless of package.json "type" (NodeNext would emit CJS
+          // here and break under jest's ESM VM → "exports is not defined").
+          module: 'ESNext',
+          moduleResolution: 'Bundler',
+          isolatedModules: true,
+          experimentalDecorators: true,
+          emitDecoratorMetadata: true,
+          verbatimModuleSyntax: false,
+        },
+      },
+    ],
+  },
+}