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/curriculum_io.py

@@ -92,7 +92,7 @@ def parse(raw: bytes) -> list[dict]:
                     'lessons': [
                         {
                             'title': _text(lesson.get('title'), 'title', f'{nwhere} › نوع {li + 1}'),
-                            'intro_text': (lesson.get('introText') or lesson.get('intro_text') or ''),
+                            'intro_text': (lesson.get('introText') or ''),
                             'example': str(lesson.get('example') or ''),
                             'steps': [str(x) for x in (lesson.get('steps') or []) if str(x).strip()][:12],
                         }