Add proper uninstall.php and extract shared Cleanup class

[yoav-cloud]

Background
Previously, the plugin had no uninstall.php. All data removal logic lived exclusively inside Deactivation::cleanup(), which was only reachable when a user chose "Remove all plugin data" from the deactivation modal. WordPress's standard uninstall hook — triggered when a user clicks Delete in the Plugins screen — was never wired up, meaning plugin data (options, post meta, term meta, custom tables, cron jobs) was left behind after a full uninstall.

What changed
uninstall.php (new)
Registers the plugin's uninstall handler with WordPress. The file guards against direct execution via WP_UNINSTALL_PLUGIN, then bootstraps the minimum plugin runtime needed for component-aware cleanup (init → plugins_loaded → setup_settings) before delegating to Cleanup::run().

php/class-cleanup.php (new)
Extracts all data-removal routines from Deactivation into a standalone Cleanup class so the same logic can be invoked from both the deactivation modal path and the uninstall path without duplication. The class:

Accepts an optional Plugin instance and Settings instance (with a fallback to $plugin->settings)
Guards every settings-dependent branch with instanceof checks so it degrades gracefully if settings are not available
Exposes a static Cleanup::run() factory entry point
Owns the CLEANING_KEY constant (previously on Deactivation), since it is Cleanup that reads and deletes this option
Adds array_unique( array_filter(...) ) around the option-key list in cleanup_options() to prevent redundant or empty-key deletes
Adds cloudinary_cleanup_event to cleanup_legacy_cron() so that the deactivation-modal's scheduled retry hook is always cleared, regardless of which code path triggered cleanup
php/class-deactivation.php (modified)
Removes the CLEANING_KEY constant (now on Cleanup) and updates both references to Cleanup::CLEANING_KEY
Removes all private cleanup methods (moved to Cleanup)
Removes the unused use imports (Global_Transformations, Storage) that were only needed by those methods
Simplifies cleanup() to: set the cleaning flag → Cleanup::run() → clear the cron hook