Audit fixes 2026-10-03: seed keeps admin edits, plan cache, refresh limit, payment and tutor guards

- seed_content no longer undoes admin work: a SeedRecord ledger (content/seed_sync.py)
  stops deleted/renamed/re-worded content from being recreated and keeps admin-edited
  lesson text; new rows in the seed files still arrive (migration content.0010)
- plan list cache is invalidated on Plan save (receiver used a stale 'v1' key)
- token refresh per-IP cap 30/hour -> 600/hour (TOKEN_REFRESH_IP_RATE)
- a failed gateway result only changes a pending order (forged NOK no longer
  overwrites refunded/canceled)
- tutor gets the correct answer only after the student has answered
- support reply notification moved to a TicketMessage post_save signal
- global daily cap on paid AI questions (AI_GLOBAL_DAILY_MAX, default 3000)
- Bazaar verify uses only the order's SKU; bazaar orders require a SKU
- /api/health warns on the committed signing key; deploy.sh saves local commits;
  400 for bad ?unit= / ?period=; account delete keeps the referrer's Conversion
- earlier today: no stub answers outside DEBUG, ?grade= removed, logged fallbacks

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

content/tests.py

@@ -796,3 +796,16 @@ class HomeGradeFallbackTests(TestCase):
 
         self.assertEqual(response.status_code, 200)
         self.assertEqual(response.data['units'], [])
+
+
+class NodeProgressUnitParamTests(TestCase):
+    def test_non_numeric_unit_is_a_400_not_a_500(self):
+        from rest_framework.test import APIClient
+
+        from accounts.authentication import issue_access_token
+        from accounts.models import User
+
+        user = User.objects.create(device_id='unit-param', is_guest=True)
+        response = APIClient().get('/api/progress/nodes?unit=abc',
+                                   HTTP_AUTHORIZATION=f'Bearer {issue_access_token(user.id)}')
+        self.assertEqual(response.status_code, 400)