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>

tests/unit/test_folder_rename_service.py

@@ -9,6 +9,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 
 from src.server.services.folder_rename_service import (
+    _cleanup_orphaned_folder,
     _compute_expected_folder_name,
     _is_series_being_downloaded,
     _parse_nfo_title_and_year,
@@ -278,6 +279,71 @@ class TestUpdateDatabasePaths:
             assert mock_episode.file_path == str(new_path)
 
 
+class TestCleanupOrphanedFolder:
+    """Tests for _cleanup_orphaned_folder."""
+
+    def test_returns_false_when_old_folder_does_not_exist(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "nonexistent"
+        new_path = tmp_path / "new"
+        result = _cleanup_orphaned_folder(old_path, new_path)
+        assert result is False
+
+    def test_deletes_empty_folder(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "empty_orphan"
+        old_path.mkdir()
+        new_path = tmp_path / "new"
+        new_path.mkdir()
+        result = _cleanup_orphaned_folder(old_path, new_path)
+        assert result is True
+        assert not old_path.exists()
+
+    def test_moves_files_and_deletes_folder(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "old_orphan"
+        old_path.mkdir()
+        new_path = tmp_path / "new"
+        new_path.mkdir()
+        file1 = old_path / "S01E01.mkv"
+        file1.write_text("episode 1")
+        file2 = old_path / "S01E02.mkv"
+        file2.write_text("episode 2")
+        result = _cleanup_orphaned_folder(old_path, new_path)
+        assert result is True
+        assert not old_path.exists()
+        assert (new_path / "S01E01.mkv").exists()
+        assert (new_path / "S01E02.mkv").exists()
+
+    def test_dry_run_does_not_delete_empty_folder(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "empty_orphan"
+        old_path.mkdir()
+        new_path = tmp_path / "new"
+        new_path.mkdir()
+        result = _cleanup_orphaned_folder(old_path, new_path, dry_run=True)
+        assert result is True
+        assert old_path.exists()
+
+    def test_dry_run_does_not_move_files(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "old_orphan"
+        old_path.mkdir()
+        new_path = tmp_path / "new"
+        new_path.mkdir()
+        file1 = old_path / "S01E01.mkv"
+        file1.write_text("episode 1")
+        result = _cleanup_orphaned_folder(old_path, new_path, dry_run=True)
+        assert result is True
+        assert old_path.exists()
+        assert not (new_path / "S01E01.mkv").exists()
+
+    def test_handles_permission_error_gracefully(self, tmp_path: Path) -> None:
+        old_path = tmp_path / "permission_denied"
+        old_path.mkdir()
+        new_path = tmp_path / "new"
+        new_path.mkdir()
+        # Simulate permission error by patching rmdir
+        with patch.object(Path, "rmdir", side_effect=PermissionError("Access denied")):
+            result = _cleanup_orphaned_folder(old_path, new_path)
+        assert result is False
+
+
 class TestValidateAndRenameSeriesFolders:
     """Integration-style tests for validate_and_rename_series_folders."""
 
@@ -459,3 +525,30 @@ class TestValidateAndRenameSeriesFolders:
         assert (anime_dir / "Show A (2020)").is_dir()
         assert d2.is_dir()
         assert d3.is_dir()
+
+    @pytest.mark.asyncio
+    async def test_dry_run_does_not_rename_folders(self, tmp_path: Path) -> None:
+        anime_dir = tmp_path / "anime"
+        anime_dir.mkdir()
+        series_dir = anime_dir / "Attack on Titan"
+        series_dir.mkdir()
+        (series_dir / "tvshow.nfo").write_text(
+            "<tvshow><title>Attack on Titan</title><year>2013</year></tvshow>"
+        )
+
+        with patch(
+            "src.server.services.folder_rename_service.settings.anime_directory",
+            str(anime_dir),
+        ), patch(
+            "src.server.services.folder_rename_service._is_series_being_downloaded",
+            return_value=False,
+        ):
+            stats = await validate_and_rename_series_folders(dry_run=True)
+
+        assert stats["scanned"] == 1
+        assert stats["renamed"] == 1
+        assert stats["skipped"] == 0
+        assert stats["errors"] == 0
+        # Original folder should still exist (not renamed in dry-run)
+        assert series_dir.is_dir()
+        assert not (anime_dir / "Attack on Titan (2013)").exists()