feat: implement OAuth 2.0 Device Authorization Grant (RFC 8628) for WebDAV/CalDAV/CardDAV

Adds full Device Authorization Grant flow so DAV clients (rclone, etc.)
can authenticate without browser-based OAuth redirects.

New files:
- Domain entity: DeviceCode with status lifecycle (pending/authorized/denied/expired)
- Port: DeviceCodeStoragePort trait (7 async methods)
- DTOs: request/response types for all device auth endpoints
- Repository: DeviceCodePgRepository (PostgreSQL implementation)
- Service: DeviceAuthService (initiate, verify, approve, deny, poll, cleanup)
- Handler: 6 HTTP endpoints (2 public + 4 protected)
- Static: device-verify.html verification page served at /device

Flow:
1. Client POST /api/auth/device/authorize → device_code + user_code
2. User opens /device?code=XXXX in browser, approves
3. Client polls POST /api/auth/device/token → receives JWT tokens
4. Client uses Bearer token with existing WebDAV/CalDAV/CardDAV middleware

Schema: auth.device_codes table + device_code_status enum added to schema.sql

Closes #152

src/interfaces/api/handlers/mod.rs

@@ -1,6 +1,7 @@
 pub mod admin_handler;
 pub mod auth_handler;
 pub mod batch_handler;
+pub mod device_auth_handler;
 pub mod caldav_handler;
 pub mod carddav_handler;
 pub mod chunked_upload_handler;