feat(05-03): calendar backend — model, crypto, source CRUD module

- Add CalendarSource Prisma model with encrypted credentials (AES-256-GCM)
- Create CalendarCryptoService with encrypt/decrypt using CALENDAR_ENCRYPTION_KEY
- Create CalendarController with source CRUD endpoints (GET/POST/PATCH/DELETE)
- Create CalendarService with ownership checks and SSRF URL validation
- Add DTOs with https-only URL validation and class-validator decorators
- Register CalendarModule in AppModule
- Install tsdav, node-ical, ews-javascript-api, @microsoft/microsoft-graph-client
- Stub provider files for Task 2 compilation

apps/api/src/calendar/calendar.module.ts

@@ -0,0 +1,33 @@
+import { Module } from '@nestjs/common';
+import { CalendarController } from './calendar.controller';
+import { CalendarService } from './calendar.service';
+import { CalendarCryptoService } from './crypto.service';
+import { CalDAVProvider } from './providers/caldav.provider';
+import { ICSProvider } from './providers/ics.provider';
+import { ExchangeProvider } from './providers/exchange.provider';
+
+/**
+ * NestJS module for calendar source management and event aggregation.
+ *
+ * Provides:
+ * - CalendarCryptoService: AES-256-GCM encryption for calendar credentials
+ * - CalendarService: Source CRUD + event aggregation across providers
+ * - CalDAVProvider: CalDAV protocol integration via tsdav
+ * - ICSProvider: ICS file fetch + parse via node-ical
+ * - ExchangeProvider: Exchange Online (Graph) + Exchange Server (EWS) integration
+ * - CalendarController: REST API for sources and events
+ *
+ * Exports CalendarService for potential use by other modules.
+ */
+@Module({
+  controllers: [CalendarController],
+  providers: [
+    CalendarService,
+    CalendarCryptoService,
+    CalDAVProvider,
+    ICSProvider,
+    ExchangeProvider,
+  ],
+  exports: [CalendarService],
+})
+export class CalendarModule {}