Remove silent fallbacks found in the 2026-10-03 audit

- /api/ai/ask: the stub answer is reachable only under DEBUG or
  RAG_FORCE_MOCK. Without RAG keys on the server it used to return 200 with
  "this is a stub, set OPENAI_API_KEY" and spend the student's quota; now 503.
- /api/home: the `grade` alias is gone and sending it is a 400 instead of a
  silent fallback to the user's own grade. No client sent it.
- activate_subscription logs ERROR when the paid order's plan no longer exists
  (it silently became 30 days).
- resolve_grade_title logs instead of `except: pass`; curriculum_io no longer
  reads an `intro_text` key no file contains.
- CLAUDE.md: "No silent fallbacks" rules for future agents.

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

ai_tutor/tests.py

@@ -1,7 +1,7 @@
 from unittest.mock import patch
 
 from django.core.cache import cache
-from django.test import TestCase
+from django.test import TestCase, override_settings
 from rest_framework.test import APIClient
 from rest_framework.throttling import ScopedRateThrottle
 
@@ -514,3 +514,18 @@ class AskAIViewGuestThrottleBypassTests(TestCase):
             response_b = self._ask(issue_access_token(user_b.id), 'user-b-key-1')
 
         self.assertEqual(response_b.status_code, 200)
+
+
+class StubNeverReachesProductionTests(TestCase):
+    """بی‌کلیدِ RAG و بیرون از DEBUG، دانش‌آموز ۵۰۳ می‌گیرد، نه متنِ «پاسخ نمونه (stub)»."""
+
+    @override_settings(DEBUG=False, OPENAI_API_KEY='', QDRANT_API_KEY='', RAG_FORCE_MOCK=False)
+    def test_missing_keys_are_a_503_and_cost_nothing(self):
+        cache.clear()
+        user = User.objects.create(device_id='device-no-keys', is_guest=True, name='')
+        response = APIClient().post(
+            '/api/ai/ask', {'userQuestion': '۲+۲ چند است؟'}, format='json',
+            HTTP_AUTHORIZATION=f'Bearer {issue_access_token(user.id)}',
+        )
+        self.assertEqual(response.status_code, 503)
+        self.assertFalse(Message.objects.filter(conversation__user_id=user.id).exists())