Hint foreign key behaviour

Currently there is no way to know what is the "on delete", "on update", "validation" behaviour of a foreign key without clicking on each foreign key.

It would be handy to have some coded overview, e.g. `[mention.id delete=cascade]`

No files, tests, or entry points are named; start with the foreign-key list and detail views shown by the issue and trace how their metadata is loaded. Done means the list provides a coded summary of on-delete, on-update, and validation behavior without opening each foreign key.

Written by the indexing model from the issue text.