Leave both copies alone when the array copy is newer than the cache copy

[Brandon-Haney]

Follow-up to #222, from @DeLo1585's point on #221.

#222 made Move to Array keep the cache copy whenever it differs in size from the array copy, which is right when a tool rewrote the cache side (a Sonarr/Radarr in-place upgrade, a Tdarr pass through /mnt/user). When a tool rewrote the array side directly, the cache copy is the stale one, and Move to Array copied it over the newer array file.

Change

Caching keeps the original modification time on both sides: the cache copy is written with copystat, and the .plexcached backup is a rename of the original. So an untouched pair has matching mtimes, and whichever side a tool rewrote ends up newer.

• If the array copy (the same-name file or the .plexcached backup) is newer than the cache copy by more than 2 seconds, Move to Array skips that file and reports "Array copy is newer than the cache copy, left both in place for review". Nothing is copied, renamed or deleted.
• Matching mtimes, including tools that preserve the original mtime, keep Keep the newer cache copy when Move to Array finds an older array copy #222's behaviour: the cache copy wins when the sizes differ.
• The 2-second tolerance covers filesystems that store modification times coarsely.
• The Move to Array confirm dialog mentions the new case, and the parallel path's byte total leaves skipped files out.

Tests

tests/test_sync_to_array_newer_cache.py gains 10 cases (sequential and parallel): a newer same-name array file and a newer backup are both left untouched, a newer cache copy still replaces the array copy, and same-mtime / within-tolerance pairs keep the cache version. Without the guard, the four "array copy is newer" cases fail.

How to test

1. Pick a file on cache with a .plexcached backup on the array.
2. Replace the backup with a different file and touch it so it is newer than the cache copy.
3. Run an audit and use Move to Array on that file: it is skipped with "Array copy is newer…", and both copies are unchanged.
4. Repeat with the cache copy changed instead (newer than the backup): the cache version is copied across as before.