Allow owning files in storage and deleting them via cascading deletes

[xixixao]

Suggested on Discord: https://discord.com/channels/1019350475847499849/1202403482930249789/1202795277132759131

This is tricky because:

1. The ID of the "owning" ent cannot be stored in _storage. This means that the edge needs to be represented in reverse (by storing the _storage ID on the owning ent), something that isn't supported yet for 1:1 edges in general.
2. The API for deleting files is different to the API for deleting ents/documents.