Let referee levels inherit from a lower level instead of relying on ordering alone

RefereeLevel.inherits_from chains levels together so a higher tier is
automatically eligible for everything a linked lower tier covers,
transitively, without hand-duplicating teams onto every level. Kept
the ordering field (still drives display order) but eligibility
everywhere (RefereeProfile.eligible_teams, events.services.referees,
the team detail page's eligible-referees list) now reads through
RefereeLevel.eligible_team_ids, which walks the inherits_from chain.
clean() rejects a loop, including an indirect one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

teams/migrations/0011_refereelevel_inherits_from.py

@@ -0,0 +1,19 @@
+# Generated by Django 6.0.6 on 2026-08-20 20:39
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('teams', '0010_alter_team_options_alter_teammembership_position'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='refereelevel',
+            name='inherits_from',
+            field=models.ForeignKey(blank=True, help_text='A referee holding this level is also eligible for everything the linked level covers (and, transitively, whatever that one inherits from).', null=True, on_delete=django.db.models.deletion.PROTECT, related_name='inherited_by', to='teams.refereelevel', verbose_name='inherits from'),
+        ),
+    ]