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_frontend_contract.py

@@ -142,8 +142,8 @@ class HomeGradeParamTests(TestCase):
     def test_grade_id_selects_that_grade(self):
         self.assertEqual(self._titles(f'?gradeId={self.b.id}'), ['فصلِ نهم'])
 
-    def test_the_old_grade_name_still_works(self):
-        self.assertEqual(self._titles(f'?grade={self.b.id}'), ['فصلِ نهم'])
+    def test_the_removed_grade_name_is_a_400_not_a_silent_fallback(self):
+        self.assertEqual(self.client.get(f'/api/home?grade={self.b.id}', **self.auth).status_code, 400)
 
     def test_without_a_parameter_the_users_own_grade_is_used(self):
         self.assertEqual(self._titles(), ['فصلِ هفتم'])