Add trial subscriptions with automatic switch to a pre-selected plan

A club with no subscription yet can be started on a short trial (e.g.
2 months) from the control panel, on a tier picked up front for what
it switches to once the trial ends -- no manual follow-up needed. The
trial is a real billed period on a dedicated trial tier, reusing the
existing invoice/grace/archive machinery unchanged; the switch happens
in open_period() itself so it fires whether reached via the scheduled
renewal command or a platform admin's manual "Open period" click.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R1gj3J1QPfP38XWpnpbFpy

billing/migrations/0003_due_is_trial_subscription_post_trial_tier_and_more.py

@@ -0,0 +1,34 @@
+# Generated by Django 6.0.6 on 2026-08-04 10:10
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('billing', '0002_subscription_auto_renew'),
+        ('club', '0017_club_season_duration_months_club_season_start'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='due',
+            name='is_trial',
+            field=models.BooleanField(default=False, help_text="This period was opened as a trial. A durable marker on the row itself -- the subscription's own trial fields are cleared once it converts.", verbose_name='trial period'),
+        ),
+        migrations.AddField(
+            model_name='subscription',
+            name='post_trial_tier',
+            field=models.ForeignKey(blank=True, help_text='The plan this club switches to automatically once its trial ends.', null=True, on_delete=django.db.models.deletion.PROTECT, related_name='+', to='billing.tier', verbose_name='post-trial tier'),
+        ),
+        migrations.AddField(
+            model_name='subscription',
+            name='trial_ends_at',
+            field=models.DateField(blank=True, help_text='Set while this club is on a trial. The tier switches to post_trial_tier the next time a period is opened after this date.', null=True, verbose_name='trial ends at'),
+        ),
+        migrations.AddConstraint(
+            model_name='subscription',
+            constraint=models.CheckConstraint(condition=models.Q(models.Q(('post_trial_tier__isnull', True), ('trial_ends_at__isnull', True)), models.Q(('post_trial_tier__isnull', False), ('trial_ends_at__isnull', False)), _connector='OR'), name='trial_fields_set_together'),
+        ),
+    ]