feat(teams): add teams app with roster and staff assignments

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>

teams/admin.py

@@ -0,0 +1,56 @@
+from django.contrib import admin
+from django.utils.translation import gettext_lazy as _
+
+from .models import Position, StaffAssignment, Team, TeamMembership
+
+
+class TeamMembershipInline(admin.TabularInline):
+    """Roster shown on the Team page."""
+
+    model = TeamMembership
+    extra = 0
+    raw_id_fields = ("member",)
+
+
+class StaffAssignmentInline(admin.TabularInline):
+    """Coaching / staff shown on the Team page."""
+
+    model = StaffAssignment
+    extra = 0
+    raw_id_fields = ("member",)
+
+
+@admin.register(Team)
+class TeamAdmin(admin.ModelAdmin):
+    list_display = ["name", "short_name", "club"]
+    list_filter = ["club"]
+    search_fields = ["name", "short_name"]
+    inlines = [TeamMembershipInline, StaffAssignmentInline]
+
+
+@admin.register(Position)
+class PositionAdmin(admin.ModelAdmin):
+    list_display = ["name", "short_name", "club", "staff_position", "ordering"]
+    list_filter = ["club", "staff_position"]
+    search_fields = ["name", "short_name"]
+    ordering = ["ordering", "name"]
+
+
+@admin.register(TeamMembership)
+class TeamMembershipAdmin(admin.ModelAdmin):
+    list_display = ["team", "member", "season", "position", "jersey_number", "is_captain", "is_alternate_captain"]
+    list_filter = ["team", "season", "position", "is_captain"]
+    search_fields = ["team__name", "member__first_name", "member__last_name"]
+    raw_id_fields = ["member"]
+    fieldsets = [
+        [None, {"fields": ["team", "season", "member", "position"]}],
+        [_("Squad"), {"fields": ["jersey_number", "is_captain", "is_alternate_captain"]}],
+    ]
+
+
+@admin.register(StaffAssignment)
+class StaffAssignmentAdmin(admin.ModelAdmin):
+    list_display = ["team", "member", "season", "position"]
+    list_filter = ["team", "season", "position"]
+    search_fields = ["team__name", "member__first_name", "member__last_name"]
+    raw_id_fields = ["member"]