Position gains management_position, distinguishing a coach/manager from other
staff (physio, kit manager). A CheckConstraint enforces that a management
position is always a staff position.
TeamMembership and StaffAssignment validate that their season and position
belong to the same club as the team.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Introduce the teams app: Team and Position (both club-scoped, with
per-club unique names), TeamMembership (season-scoped roster with jersey
number + captain flags, unique member and jersey per team/season), and
StaffAssignment (coaching/staff, filtered to staff positions). All
uniqueness expressed as UniqueConstraints.
Register every model in the admin (with roster + staff inlines on Team)
and cover the models with tests. Fix two model bugs surfaced by the
system check: StaffAssignment.position reused TeamMembership's reverse
accessor (Position.team_memberships) and duplicated its constraint name.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
