Security review fixes: admin permissions, guest sessions, payment edge cases, assistant leaks

- Admin: refund/cancel/mark-resolved/Bazaar-link actions need change permission; question and
  curriculum import need add/delete; content browser needs view_question; guest device ids are
  shown to superusers only.
- Accounts: registering a guest's phone revokes the guest's earlier sessions; self-deletion keeps
  Redemption, UserOffer and the phone's OTP rows; the public demo guest cannot be deleted
  (DEMO_GUEST_DEVICE_IDS); display-name contact filter covers any separator and social handles.
- Payment: callback without Authority is 400; a second Bazaar purchase on a paid order is 409;
  orders snapshot credits/hearts/days at creation (payment.0011).
- Assistant: a broken diagram no longer makes a streamed answer free; a late off-topic marker
  refuses the answer (not stored, not cached); a disconnect saves the partial answer; only
  KNOWN_COLLECTIONS are searched.
- Infra: secret-key file is created atomically (workers no longer split keys); no default
  INTERNAL_SERVICE_KEY; Kavenegar key and Bazaar token kept out of error logs; Swagger/ReDoc
  pinned; schema throttled; docker compose bound to 127.0.0.1; non-ASCII digit ids are 400;
  ticket text capped at 5000.
- docs/security-before-production.md: remaining open items 24-31 and the guard tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

payment/migrations/0011_order_plan_snapshot.py

@@ -0,0 +1,33 @@
+# Generated by Django 5.2.17 on 2026-10-05 01:06
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('payment', '0010_order_cancel_refund'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='duration_days',
+            field=models.PositiveIntegerField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='hearts_days',
+            field=models.PositiveIntegerField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='question_credits',
+            field=models.PositiveIntegerField(blank=True, null=True),
+        ),
+        migrations.AlterField(
+            model_name='transaction',
+            name='gateway_ref_id',
+            field=models.CharField(blank=True, max_length=300),
+        ),
+    ]