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/package.json

@@ -9,6 +9,7 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
+    "@microsoft/microsoft-graph-client": "3.0.7",
     "@nestjs-modules/mailer": "^2.3.7",
     "@nestjs/common": "^11.0.0",
     "@nestjs/config": "^4.0.0",
@@ -24,13 +25,16 @@
     "class-transformer": "^0.5.1",
     "class-validator": "^0.15.1",
     "cookie-parser": "^1.4.7",
+    "ews-javascript-api": "0.15.3",
     "ldapts": "^8.1.8",
+    "node-ical": "0.26.1",
     "nodemailer": "^9.0.1",
     "passport": "^0.7.0",
     "passport-jwt": "^4.0.1",
     "passport-local": "^1.0.0",
     "reflect-metadata": "^0.2.0",
-    "rxjs": "^7.0.0"
+    "rxjs": "^7.0.0",
+    "tsdav": "2.2.2"
   },
   "devDependencies": {
     "@nestjs/cli": "^11.0.0",