Build out Positions CRUD and rework the Roles page

Positions had list/create/edit already stubbed as list-only; give it real
forms, with a check mirroring the management_position_implies_staff_position
constraint so a bad combination reads as a form error, not a 500. Roles now
groups by role (excluding the MEMBER everyone holds automatically, which was
just noise), grants via a modal instead of a separate page, and its member
picker is a small typeahead combobox instead of a long native <select>.

teams/migrations/0006_alter_position_ordering.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-08-03 15:58
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('teams', '0005_alter_position_management_position_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='position',
+            name='ordering',
+            field=models.PositiveSmallIntegerField(default=0, help_text="Lower numbers are listed first (e.g. on a team's roster). Positions with the same number are ordered by name.", verbose_name='ordering'),
+        ),
+    ]