v0.4.0

Media files can now be protected from deletion

• New MediaUsageRegistry singleton: any module registers a check from its service provider's boot() and returns a human-readable reason when a file must not be deleted. The media module stays free of knowledge about who uses its files.
• The guard sits on the Media model (deleting event) and throws MediaInUseException, so every deletion path is covered — library screen, bulk delete, commands, tinker.
• The media library deletes the ROW first and the FILE second, and shows the reason instead of refusing silently. Previously a refused deletion could leave a record pointing at a file that was already gone.
• A bulk delete removes the free files and keeps the claimed ones selected; a check that throws is reported and falls through instead of making the file undeletable.