A file node points at one media row and owns it the way a document owns
what it embeds, so folder deletes, the media browser and exports keep
working. POST /api/nodes/{id}/files streams uploads into a folder;
imports turn non-markdown files into file nodes; exports put them at
their tree path. The browser previews images, video, audio and text,
offers the rest as a download, and takes files dropped on the list.
Migration 0010 widens the kind CHECK by editing the stored schema, as
SQLite documents for loosening a constraint: sqlx runs migrations in a
transaction, where foreign keys cannot be switched off for a rebuild.
