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/dto/calendar-events-query.dto.ts

@@ -0,0 +1,16 @@
+import { IsDateString, IsOptional } from 'class-validator';
+
+/**
+ * DTO for querying aggregated calendar events.
+ *
+ * Default window: now to now + 30 days (applied in CalendarService).
+ */
+export class CalendarEventsQueryDto {
+  @IsOptional()
+  @IsDateString()
+  from?: string;
+
+  @IsOptional()
+  @IsDateString()
+  to?: string;
+}