fix(calendar): change owner_id from String to Uuid type

The database schema defines owner_id as UUID, but the Calendar entity
stored it as String. This caused a type mismatch error when creating
calendars via CalDAV clients like DAVx:

  column "owner_id" is of type uuid but expression is of type text

Changes:
- Change Calendar.owner_id from String to Uuid
- Update new() and with_id() to accept Uuid
- Update owner_id() getter to return &Uuid
- Update belongs_to() to accept &Uuid
- Update calendar_storage_adapter to pass Uuid directly
- Update tests to use Uuid::new_v4()

Fixes #200

src/infrastructure/adapters/calendar_storage_adapter.rs

@@ -49,7 +49,7 @@ impl CalendarStoragePort for CalendarStorageAdapter {
         dto: CreateCalendarDto,
         owner_id: Uuid,
     ) -> Result<CalendarDto, DomainError> {
-        let calendar = Calendar::new(dto.name, owner_id.to_string(), dto.description, dto.color)?;
+        let calendar = Calendar::new(dto.name, owner_id, dto.description, dto.color)?;
 
         let created = self.calendar_repository.create_calendar(calendar).await?;
         Ok(CalendarDto::from(created))