Fix: chown directories created during caching to PUID/PGID (#178)

[Brandon-Haney]

Problem

On Unraid, PlexCache-D creates new top-level library folders (e.g. /mnt/user/data/media/tv-shows/) as root:root with drwxr-xr-x (0755) instead of nobody:users drwxrwxrwx. Because 0755 doesn't grant write access to non-root users, this blocks Sonarr/Radarr from creating new subdirectories inside those folders — imports fail with Access to the path '...' is denied.

Fixes #178.

Root cause

The container runs as root (so it can move files of any ownership) and compensates by chown-ing what it creates to PUID/PGID. File copies already do this via copy_file_with_permissions(), but several move/copy destination directory creations used a raw os.makedirs(), which — running as root — leaves new directories owned by root:root with a umask-derived mode. On a multi-disk array, a /mnt/user0/ write can land the folder chain on a disk where the library root doesn't exist yet, recreating it (and its parents) as root.

Fix

• Add a shared create_dir_with_ownership() helper in core/system_utils.py that creates a directory tree and chowns every newly created level to PUID/PGID (or the source file's owner when PUID/PGID are unset), best-effort and Linux-only — matching how file copies are already handled.
• Route the move/copy destination directory creation through it across the caching, eviction, upgrade, restore, symlink, and audit paths.
• FileUtils.create_directory_with_permissions() now delegates to the helper, so there is a single implementation shared with callers that have no FileUtils instance (the web service layer).

Testing

• Added TestCreateDirWithOwnership (4 tests): nested-tree creation, no-op when the path already exists, PUID/PGID applied to every newly created level (pre-existing ancestors left untouched), and fallback to the source file's owner when PUID is invalid/unset.
• Full suite passes.

Manual verification (Unraid, container with PUID=99 / PGID=100)

1. Trigger a run that moves a watched item back to the array where its show folder doesn't currently exist on the array (or restore a file with no .plexcached backup) — anything that makes PlexCache create a new array folder.
2. Confirm the newly created library/show/season folders are nobody:users drwxrwxrwx, not root:root drwxr-xr-x.
3. Confirm Sonarr/Radarr can import a new episode into that folder.

Note: only newly created directories are governed by this change; folders already created as root:root by prior runs are not retroactively fixed (use "Docker Safe New Permissions" once to clean those up).