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>

subscription/services.py

@@ -1,3 +1,5 @@
+import logging
+
 from django.db import models, transaction
 from django.utils import timezone
 
@@ -5,6 +7,8 @@ from statuslog.signals import status_changed
 
 from .models import HeartsPass, QuestionCredit, Subscription
 
+logger = logging.getLogger(__name__)
+
 
 def has_unlimited_hearts(user_id: int) -> bool:
     """
@@ -126,6 +130,11 @@ def activate_subscription(subscription_id: int, plan_id: int | None = None):
             return None
         old_status = subscription.status
         plan = Plan.objects.filter(pk=plan_id).first() if plan_id else None
+        if plan_id and plan is None:
+            # پلنِ سفارشِ پرداخت‌شده پاک شده؛ جایگزین (پلنِ Subscription یا ۳۰ روز)
+            # ممکن است کمتر از آن‌چه خریده شده باشد. کاربر بی‌اشتراک نمی‌ماند، ولی
+            # باید دیده شود. پلن را پاک نکنید؛ on_sale=False کنید.
+            logger.error('activate_subscription: پلنِ %s برای اشتراکِ %s پیدا نشد', plan_id, subscription_id)
         plan = plan or subscription.plan
         duration_days = plan.duration_days if plan else 30
         now = timezone.now()