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/integration/test_series_parsing_edge_cases.py

@@ -495,6 +495,20 @@ class TestNameWithYearProperty:
         assert "(2013)" in sanitized
         assert "Attack on Titan" in sanitized
 
+    def test_name_with_year_does_not_duplicate(self):
+        """Test that name_with_year doesn't duplicate year."""
+        serie = Serie(
+            key="eighty-six",
+            name="86 Eighty Six (2021)",
+            site="aniworld.to",
+            folder="86 Eighty Six (2021)",
+            episodeDict={},
+            year=2021
+        )
+        
+        assert serie.name_with_year == "86 Eighty Six (2021)"
+        assert serie.name_with_year.count("(2021)") == 1
+
 
 class TestEnsureFolderWithYear:
     """Test Serie.ensure_folder_with_year method."""