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/infrastructure/repositories/mod.rs

@@ -3,6 +3,6 @@ pub mod pg;
 
 // Re-exportar para facilitar acceso
 pub use pg::{
-    FileBlobReadRepository, FileBlobWriteRepository, FolderDbRepository, SessionPgRepository,
-    TrashDbRepository, UserPgRepository,
+    DeviceCodePgRepository, FileBlobReadRepository, FileBlobWriteRepository,
+    FolderDbRepository, SessionPgRepository, TrashDbRepository, UserPgRepository,
 };