Fix temp files generated during the upload process not being cleaned up quickly enough. This included
downloaded files, generated preview images, and Ugoira video conversions.
Before we relied on `Tempfile` cleaning up files automatically. But this only happened when the
Tempfile object was garbage collected, which could take a long time. In the meantime we could have
hundreds of megabytes of temp files hanging around.
The fix is to explicitly close temp files when we're done with them. But the standard `Tempfile`
class doesn't immediately delete the file when it's closed. So we also have to introduce a
Danbooru::Tempfile wrapper that deletes the tempfile as soon as it's closed.
Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.
Fix bug where pruning uploads failed because the rclone binary couldn't
be found. The upload pruner runs under cron, which has a fixed default
$PATH of "/bin:/usr/bin", but in production rclone is installed under
/usr/local/bin. This caused the upload pruner to fail, which prevented
the rest of daily maintenance from running.
