v6.5.6: feat(files): Remove empty directories left by orphan sweep

File names are often split into subdirectories by ID. Previously, the `files:delete-orphans` command would leave behind a skeleton of empty directories after removing orphaned files.

This change extends the command to recursively remove any directories that become empty as a result of the file cleanup. Directories that still contain valid files are preserved, and the configured data root directories themselves are never touched. This operation is only performed when the `--exec` flag is used.