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>

content/tests.py

@@ -587,7 +587,7 @@ class HomeViewScalabilityTests(TestCase):
 
     def _home(self, grade_id):
         return self.client.get(
-            f'/api/home?grade={grade_id}', HTTP_AUTHORIZATION=f'Bearer {self.token}')
+            f'/api/home?gradeId={grade_id}', HTTP_AUTHORIZATION=f'Bearer {self.token}')
 
     def test_query_count_is_flat_regardless_of_node_count(self):
         g_small = self._grade_with_nodes(2)
@@ -726,7 +726,7 @@ class ReadThrottleTests(TestCase):
         codes = set()
         for _ in range(122):
             codes.add(self.client.get(
-                f'/api/home?grade={self.grade.id}',
+                f'/api/home?gradeId={self.grade.id}',
                 HTTP_AUTHORIZATION=f'Bearer {self.token}').status_code)
         self.assertIn(429, codes)
 
@@ -774,7 +774,7 @@ class HomeGradeFallbackTests(TestCase):
         self.user.grade_id = self.grade.id
         self.user.save(update_fields=['grade_id'])
 
-        response = self._home(f'?grade={other.id}')
+        response = self._home(f'?gradeId={other.id}')
 
         self.assertEqual(response.data['units'][0]['title'], 'فصل مثلثات')
 
@@ -785,7 +785,7 @@ class HomeGradeFallbackTests(TestCase):
         self.assertEqual(response.data['units'], [])
 
     def test_unknown_explicit_grade_is_still_404(self):
-        self.assertEqual(self._home('?grade=999999').status_code, 404)
+        self.assertEqual(self._home('?gradeId=999999').status_code, 404)
 
     def test_a_deleted_saved_grade_degrades_to_an_empty_map(self):
         # پایه‌ای که کاربر ذخیره کرده حذف شده — این خطای کاربر نیست.