Add orphaned folder cleanup after rename

- Add _cleanup_orphaned_folder() to delete/move old folder contents after rename
- Empty folders: delete directly via rmdir()
- Non-empty folders: move contents to new path, then delete old folder
- Handle PermissionError and OSError gracefully with logging
- Add dry_run parameter to preview changes without applying them
- Add --dry-run support to validate_and_rename_series_folders()
- Add unit tests for _cleanup_orphaned_folder and dry-run mode
- All 66 related tests pass

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/server/services/folder_rename_service.py

@@ -138,6 +138,113 @@ def _cleanup_stale_files_after_rename(new_path: Path, new_name: str) -> None:
             )
 
 
+def _cleanup_orphaned_folder(old_path: Path, new_path: Path, dry_run: bool = False) -> bool:
+    """Clean up orphaned folder after successful rename.
+
+    After a folder is successfully renamed to new_path, this function checks
+    if the old_path still exists (orphaned folder) and removes it. If the
+    old folder contains files, they are moved to new_path before deletion.
+
+    Args:
+        old_path: The original folder path before rename.
+        new_path: The new folder path after rename.
+        dry_run: If True, only log actions without executing them.
+
+    Returns:
+        True if old folder was cleaned up (or would be in dry-run mode),
+        False if old folder does not exist or cleanup failed.
+    """
+    if not old_path.exists():
+        logger.debug(
+            "Old folder does not exist, no cleanup needed: %s", old_path
+        )
+        return False
+
+    # Check if folder is empty
+    try:
+        contents = list(old_path.iterdir())
+    except PermissionError as exc:
+        logger.warning(
+            "Permission denied accessing old folder %s: %s", old_path, exc
+        )
+        return False
+    except OSError as exc:
+        logger.warning(
+            "OS error accessing old folder %s: %s", old_path, exc
+        )
+        return False
+
+    if not contents:
+        # Empty folder — delete it
+        if dry_run:
+            logger.info(
+                "[DRY-RUN] Would delete empty orphaned folder: %s", old_path
+            )
+            return True
+        try:
+            old_path.rmdir()
+            logger.info("Deleted empty orphaned folder: %s", old_path)
+            return True
+        except PermissionError as exc:
+            logger.warning(
+                "Permission denied deleting folder %s: %s", old_path, exc
+            )
+            return False
+        except OSError as exc:
+            logger.warning(
+                "OS error deleting folder %s: %s", old_path, exc
+            )
+            return False
+
+    # Folder has contents — move files to new_path then delete
+    if dry_run:
+        logger.info(
+            "[DRY-RUN] Would move %d files from orphaned folder %s to %s",
+            len(contents), old_path, new_path
+        )
+        for item in contents:
+            logger.info("[DRY-RUN] Would move: %s → %s", item, new_path / item.name)
+        logger.info("[DRY-RUN] Would then delete orphaned folder: %s", old_path)
+        return True
+
+    files_moved = 0
+    errors = 0
+    for item in contents:
+        try:
+            dest = new_path / item.name
+            item.rename(dest)
+            logger.debug("Moved %s → %s", item, dest)
+            files_moved += 1
+        except PermissionError as exc:
+            logger.warning(
+                "Permission denied moving %s: %s", item, exc
+            )
+            errors += 1
+        except OSError as exc:
+            logger.warning(
+                "OS error moving %s: %s", item, exc
+            )
+            errors += 1
+
+    if files_moved > 0:
+        logger.info(
+            "Moved %d files from orphaned folder to %s",
+            files_moved, new_path
+        )
+
+    # Delete the now-empty old folder
+    try:
+        old_path.rmdir()
+        logger.info("Deleted orphaned folder after moving contents: %s", old_path)
+        return errors == 0
+    except OSError as exc:
+        logger.warning(
+            "Could not delete orphaned folder %s (may not be empty): %s",
+            old_path, exc
+        )
+        return False
+
+
 async def _update_database_paths(
     old_folder: str,
     new_folder: str,
@@ -234,7 +341,7 @@ async def _update_database_paths(
         )
 
 
-async def validate_and_rename_series_folders() -> Dict[str, int]:
+async def validate_and_rename_series_folders(dry_run: bool = False) -> Dict[str, int]:
     """Validate and rename series folders to match NFO metadata.
 
     Iterates over every subfolder in ``settings.anime_directory`` that
@@ -249,6 +356,10 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
     Skips folders where title or year is missing/empty. Logs every
     rename action.
 
+    Args:
+        dry_run: If True, simulate rename operations without actually
+            moving folders or updating the database.
+
     Returns:
         Dictionary with counts:
         - ``"scanned"``: total folders scanned
@@ -267,6 +378,9 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
         )
         return {"scanned": 0, "renamed": 0, "skipped": 0, "errors": 0}
 
+    if dry_run:
+        logger.info("Running in DRY-RUN mode — no changes will be made")
+
     stats = {"scanned": 0, "renamed": 0, "skipped": 0, "errors": 0}
 
     for series_dir in sorted(anime_dir.iterdir()):
@@ -328,7 +442,17 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
             stats["errors"] += 1
             continue
 
+        if dry_run:
+            logger.info(
+                "[DRY-RUN] Would rename folder: '%s' → '%s'",
+                current_name,
+                expected_name,
+            )
+            stats["renamed"] += 1
+            continue
+
         try:
+            old_path = series_dir
             series_dir.rename(expected_path)
             logger.info(
                 "Renamed folder: '%s' → '%s'", current_name, expected_name
@@ -341,6 +465,9 @@ async def validate_and_rename_series_folders() -> Dict[str, int]:
             # Clean up stale/legacy files after successful rename
             _cleanup_stale_files_after_rename(expected_path, expected_name)
 
+            # Clean up orphaned folder if old path still exists
+            _cleanup_orphaned_folder(old_path, expected_path, dry_run=False)
+
         except PermissionError as exc:
             logger.error(
                 "Permission denied renaming '%s' → '%s': %s",