Apply the same duplicate-year prevention logic to additional code paths:
- Serie.name_with_year property: skip adding year suffix if name already ends with it
- add_series API endpoint: avoid duplicating year in folder_name_with_year
- Add integration test for Serie.name_with_year idempotency
- Add API test for add_series endpoint year deduplication
Complements the folder_rename_service fix for comprehensive coverage.