feat(teams): management positions and cross-club validation

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>

teams/migrations/0003_position_management_position_implies_staff_position.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-07-13 07:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0009_clubrole'),
+        ('teams', '0002_position_management_position'),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='position',
+            constraint=models.CheckConstraint(condition=models.Q(('management_position', False), ('staff_position', True), _connector='OR'), name='management_position_implies_staff_position'),
+        ),
+    ]