Add account self-delete and subscription lifecycle sync (#34)

Soft-delete DELETE /api/user/ for authenticated users (hide conversations,
blacklist tokens, block staff self-delete). Sync Stripe portal cancel/change
via subscription.updated/deleted webhooks and expose cancel_at_period_end for
Account UI (chat_web_app#75 companion).

llm_be/finance/migrations/0003_subscription_cancel_period_fields.py

@@ -0,0 +1,30 @@
+# Generated by Django 6.0 on 2026-08-01 19:11
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("finance", "0002_subscription_plans_quotas"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="usersubscription",
+            name="cancel_at_period_end",
+            field=models.BooleanField(
+                default=False,
+                help_text="Stripe: subscription will cancel at current_period_end.",
+            ),
+        ),
+        migrations.AddField(
+            model_name="usersubscription",
+            name="current_period_end",
+            field=models.DateTimeField(
+                blank=True,
+                help_text="Stripe billing period end (access remains until then when canceling).",
+                null=True,
+            ),
+        ),
+    ]