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/core/entities/series.py

@@ -271,7 +271,10 @@ class Serie:
             'Dororo (2025)'
         """
         if self._year:
-            return f"{self._name} ({self._year})"
+            year_suffix = f" ({self._year})"
+            if self._name.endswith(year_suffix):
+                return self._name
+            return f"{self._name}{year_suffix}"
         return self._name
 
     @property