fix(folder-rename): prevent duplicate year suffixes in series folder names

Use regex to strip all trailing year suffixes before adding the canonical
one, preventing duplication like 'Show (2021) (2021) (2021)'.

- Add regex pattern (\s*\(\d{4}\))+\s*$ to remove all existing year suffixes
- Ensure idempotent behavior across multiple folder rename runs
- Add 7 unit tests covering the bug cases and edge scenarios

Fixes: 86 Eighty Six (2021) (2021)..., Alma-chan (2025) (2025)...

src/server/services/folder_rename_service.py

@@ -66,6 +66,9 @@ def _parse_nfo_title_and_year(nfo_path: Path) -> Tuple[Optional[str], Optional[s
 def _compute_expected_folder_name(title: str, year: str) -> str:
     """Compute the expected folder name from title and year.
 
+    Removes any existing year suffixes (e.g., "(2021)") before adding the
+    canonical one to prevent duplication across multiple folder rename runs.
+
     Args:
         title: Series title from NFO.
         year: Release year from NFO.
@@ -73,7 +76,15 @@ def _compute_expected_folder_name(title: str, year: str) -> str:
     Returns:
         Sanitised folder name in the format ``"{title} ({year})"``.
     """
-    raw_name = f"{title} ({year})"
+    import re
+
+    # Remove all trailing year suffixes to prevent duplication.
+    # This handles cases where the title already contains one or more years.
+    # Regex pattern: matches one or more " (YYYY)" at the end of the string
+    clean_title = re.sub(r'(\s*\(\d{4}\))+\s*$', '', title).strip()
+
+    year_suffix = f" ({year})"
+    raw_name = f"{clean_title}{year_suffix}"
     return sanitize_folder_name(raw_name)