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/create-calendar-source.dto.ts

@@ -0,0 +1,47 @@
+import {
+  IsBoolean,
+  IsHexColor,
+  IsIn,
+  IsNotEmpty,
+  IsOptional,
+  IsString,
+  IsUrl,
+} from 'class-validator';
+
+/**
+ * DTO for creating a new calendar source.
+ *
+ * Security:
+ * - T-05-11: URL restricted to https only (SSRF mitigation)
+ * - T-05-10: password is plaintext in transit (over TLS), encrypted at rest by CryptoService
+ */
+export class CreateCalendarSourceDto {
+  @IsString()
+  @IsNotEmpty()
+  name!: string;
+
+  @IsIn(['caldav', 'ics', 'exchange'])
+  type!: string;
+
+  @IsUrl(
+    { protocols: ['https'], require_protocol: true },
+    { message: 'URL must use HTTPS protocol' },
+  )
+  url!: string;
+
+  @IsOptional()
+  @IsString()
+  username?: string;
+
+  @IsOptional()
+  @IsString()
+  password?: string;
+
+  @IsOptional()
+  @IsIn(['ews', 'graph'])
+  exchangeMode?: string;
+
+  @IsOptional()
+  @IsHexColor()
+  color?: string;
+}