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>

rag/services/lesson_context.py

@@ -83,3 +83,24 @@ def resolve_from_lesson(lesson_id) -> dict:
         unit_title, grade_title,
     )
     return {'gradeTitle': grade_title}
+
+
+def resolve_from_grade(grade_id) -> dict:
+    """
+    {'subject', 'gradeTitle'} برای وقتی که هیچ درسی در کار نیست.
+
+    صفحه‌ی «از پی بپرس» از نوارِ پایینِ اپ باز می‌شود، نه از داخلِ یک درس؛
+    پس نه lessonId دارد نه subject. بدونِ این تابع retrieval کلاً رد می‌شد و
+    همه‌ی جواب‌های آن صفحه عمومی بودند. هر پایه دقیقاً یک کتابِ ریاضی دارد،
+    پس پایه‌ی خودِ کاربر برای انتخابِ کتاب کافی‌ست؛ فصل معلوم نیست و جستجو
+    روی کلِ همان کتاب انجام می‌شود.
+    """
+    from .config import resolve_grade_title
+
+    grade_title = resolve_grade_title(grade_id)
+    if not grade_title:
+        return {}
+    for subject, grade in MATH_CHAPTERS:
+        if grade == grade_title:
+            return {'subject': subject, 'gradeTitle': grade_title}
+    return {'gradeTitle': grade_title}