fix: prevent duplicate year suffixes in series name and folder creation

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.

tests/api/test_anime_endpoints.py

@@ -334,6 +334,25 @@ async def test_add_series_sanitizes_folder_name(authenticated_client):
     assert "?" not in folder
 
 
+@pytest.mark.asyncio
+async def test_add_series_does_not_duplicate_year(authenticated_client):
+    """Test that add_series doesn't duplicate year when name already contains it."""
+    response = await authenticated_client.post(
+        "/api/anime/add",
+        json={
+            "link": "https://aniworld.to/anime/stream/eighty-six",
+            "name": "86 Eighty Six (2021)"
+        }
+    )
+    
+    assert response.status_code == 202
+    data = response.json()
+    
+    # Folder should contain year only once
+    folder = data["folder"]
+    assert folder.count("(2021)") == 1
+
+
 @pytest.mark.asyncio
 async def test_add_series_returns_missing_episodes(authenticated_client):
     """Test that add_series returns loading progress info."""