team/season aren't TeamMembershipForm fields (the view sets them from
the URL, not user input), so Django's own validate_unique() silently
excludes both of them -- and with them, the whole
unique_jersey_number_per_team_per_season constraint. A clashing jersey
number reached the database unrejected and came back as a raw
IntegrityError. form.clean() now checks it by hand, same as
PositionForm already does for its own check constraint; the four
roster/staff save paths also get a try/except IntegrityError backstop
so any future gap degrades to an error banner instead of a 500.
Bernard Siebens
ab1d34cd0a