Make chat_backend migrations a single chain (#77)

## Summary
- Closes #76.
- `0034_waitlistentry` and `0034_customuser_use_conversation_context` both depended on `0033_agentrun_agentstep`, so the graph had two leaves.
- Waitlist is now `0035_waitlistentry` and depends on `0034_customuser_use_conversation_context`. One leaf: `0035_waitlistentry`.

## Deploy note
If `django_migrations` already has a row named `0034_waitlistentry`, rename it before migrate:

```sql
UPDATE django_migrations
SET name = '0035_waitlistentry'
WHERE app = 'chat_backend' AND name = '0034_waitlistentry';
```

Then run migrate. A database that never applied `0034_waitlistentry` can migrate as usual.

## Test plan
- [x] `chat_backend.tests.test_migration_graph` asserts a single leaf and the new parent.
- [x] `chat_backend.tests.test_public_marketing` still passes.
- [ ] On a database that already applied both 0034 siblings, run the rename SQL, then `migrate`, and confirm no `InconsistentMigrationHistory`.

Reviewed-on: #77

llm_be/chat_backend/migrations/0035_waitlistentry.py

@@ -7,7 +7,7 @@ from django.db import migrations, models
 class Migration(migrations.Migration):
 
     dependencies = [
-        ("chat_backend", "0033_agentrun_agentstep"),
+        ("chat_backend", "0034_customuser_use_conversation_context"),
     ]
 
     operations = [

llm_be/chat_backend/tests/test_migration_graph.py

@@ -0,0 +1,14 @@
+from django.db import connection
+from django.db.migrations.loader import MigrationLoader
+from django.test import TestCase
+
+
+class MigrationGraphTests(TestCase):
+    def test_chat_backend_has_a_single_leaf(self):
+        loader = MigrationLoader(connection, ignore_no_migrations=True)
+        leaves = [name for app, name in loader.graph.leaf_nodes() if app == "chat_backend"]
+        self.assertEqual(leaves, ["0035_waitlistentry"])
+
+        node = loader.graph.nodes[("chat_backend", "0035_waitlistentry")]
+        parents = [name for app, name in node.dependencies if app == "chat_backend"]
+        self.assertEqual(parents, ["0034_customuser_use_conversation_context"])