Subjects per grade (math, calculus, geometry, stats, discrete), 5 new textbooks, 202 new islands, long islands split, hand-written figures and lesson texts for the new books

- content.Subject + Unit.subject, /api/home?subject=, tutor grounded in the chosen book
- 5 new books ingested; ~1150 hand-written questions, islands named by hand
- node_split: islands over 7 lessons split into parts of <=6 (lesson ids and progress kept)
- Lesson.figures hand-authored and visually checked for 653 lessons of the new books;
  gen_lesson_figures got --subject and survives network errors
- seed files re-exported

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

content/tests_seed_content.py

@@ -21,7 +21,7 @@ CURRICULUM = {
     'grades': [{
         'title': 'پایه نهم', 'icon': 'grade-9', 'color': '#FF9800',
         'units': [{
-            'title': 'فصل ۴: توان و ریشه', 'order': 4,
+            'subject': 'ریاضی', 'title': 'فصل ۴: توان و ریشه', 'order': 4,
             'nodes': [{
                 'title': 'توان صحیح', 'order': 1,
                 'lessons': [