let a finished island quiz open the next island, and ground ask-AI in the student's book

The frontend plays node quizzes, but a node only completed once every one of its
8-13 lessons was completed separately, so students were stuck after the first
island of each unit. POST /api/nodes/{id}/quiz/complete now completes the node
(any score; score only sets stars/xp) and returns nextNodeId.

Also closes the other backend gaps found while auditing the frontend:
- /api/ai/ask without lessonId now picks the book from the user's grade instead
  of skipping retrieval, so the "ask Pi" tab is grounded in the textbook.
- guest sessions return a refreshToken; they used to go silently 401 after 1h.
- lesson_complete throttle 10/hour -> 120/hour (a lesson here is 1-2 questions).
- progressPercent is per grade, not over all 2,617 lessons of every grade.
- a one-year plan (299,000 toman) is created by migration when no plan exists.

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

ai_tutor/views.py

@@ -287,6 +287,21 @@ class AskAIView(APIView):
                 conversation.lesson_number = lesson_number
                 conversation.save(update_fields=['subject', 'lesson_number'])
 
+        # نه درسی آمده نه subject — حالتِ عادیِ صفحه‌ی «از پی بپرس» که از نوارِ
+        # پایین باز می‌شود. پایه‌ی ذخیره‌شده‌ی خودِ کاربر کتاب را معلوم می‌کند؛
+        # بدونِ این، جواب‌های آن صفحه هیچ‌وقت به کتاب مستند نمی‌شدند.
+        if not subject:
+            from rag.services.lesson_context import resolve_from_grade
+
+            grade_id = grade_id or getattr(user, 'grade_id', None)
+            derived = resolve_from_grade(grade_id)
+            subject = derived.get('subject', '')
+            grade_title = grade_title or derived.get('gradeTitle', '')
+            if subject and not conversation.subject:
+                conversation.subject = subject
+                conversation.grade_id = conversation.grade_id or grade_id
+                conversation.save(update_fields=['subject', 'grade_id'])
+
         # --- مسیرِ استریم (SSE) --- فقط وقتی stream=true و RAG پیکربندی‌شده.
         # قراردادِ JSONِ غیراستریم دست‌نخورده می‌ماند؛ Swagger که stream
         # نمی‌فرسته همون مسیرِ قبلی رو می‌گیره.