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.

src/server/api/anime.py

@@ -730,7 +730,11 @@ async def add_series(
         
         # Create folder name with year if available
         if year:
-            folder_name_with_year = f"{name} ({year})"
+            year_suffix = f" ({year})"
+            if name.endswith(year_suffix):
+                folder_name_with_year = name
+            else:
+                folder_name_with_year = f"{name}{year_suffix}"
         else:
             folder_name_with_year = name