print_archives.library_file_id -> library_files.folder_id ->
library_folders.archive_id -> print_archives. Three nullable SET NULL
links, each reasonable alone, that together made a loop
metadata.sorted_tables could not sort: it dropped those edges, warned on
every backup and every restore, and could return an order placing a
child before its parent -- which once imported library_files ahead of
library_folders and killed a restore on a ForeignKeyViolation.
The restore no longer depends on that order (it strips every foreign key
before importing and adds them back after), but the backup export sorts
the same way, and the warning ends with "may raise an error in a future
release" -- which would break backup and restore on one upgrade.
Marking one edge use_alter removes it from the sort graph, not from the
database: PostgreSQL emits it as ALTER TABLE ADD CONSTRAINT, as it
already did for every constraint on these three tables, and SQLite
inlines it into CREATE TABLE, so ON DELETE SET NULL holds on both.
Verified against PostgreSQL 16 and SQLite.
