fix(phys8): закрытие критических проблем ревью — миграции, ✓→✓, ConvectionSim

- Удалены legacy миграции 009/010/015 (создавали несуществующие physics-8-thermal/electro/optics)
- 037_physics_8_hub.sql сделана self-sufficient: INSERT OR IGNORE родителя + UPDATE
- 7 шт. literal ✓ заменены на ✓ в physics_8_lab.html (правило проекта)
- В phys.js добавлен createConvectionSim — главный визуал §4 (тороидальный поток частиц)
  закрытие плана Phase 0 Физики 8

backend/src/db/migrations/037_physics_8_hub.sql

@@ -1,19 +1,27 @@
--- Physics 8 hub migration.
--- Rebuilds physics-8 as a full 3-chapter + lab textbook in the style of physics-10:
+-- Physics 8 hub migration (self-sufficient).
+-- Creates / rebuilds physics-8 as a full 3-chapter + lab textbook in the style of physics-10:
 --   physics-8           (hub, html_path = physics_8_hub.html)
 --   physics-8-ch1       (Тепловые явления, §§1–11)         → physics_8_ch1.html
 --   physics-8-ch2       (Электромагнитные явления, §§12–31) → physics_8_ch2.html
 --   physics-8-ch3       (Световые явления, §§32–40)         → physics_8_ch3.html
 --   physics-8-lab       (Лабораторный практикум, 7 ЛР)      → physics_8_lab.html
 --
--- Replaces the old legacy children created in migration 015
--- (physics-8-thermal / physics-8-electro / physics-8-optics), which pointed
--- to monolithic legacy files. Author left empty per project policy.
+-- This migration is the single source of truth for physics-8 textbook structure.
+-- It is idempotent and does not depend on any prior physics-8 migration.
 
--- 1. Remove legacy children (HTML files are kept on disk as backup, just unlinked from DB).
+-- 1. Remove legacy children if they ever existed (no-op on fresh DBs).
 DELETE FROM textbooks WHERE slug IN ('physics-8-thermal', 'physics-8-electro', 'physics-8-optics');
 
--- 2. Update the parent physics-8 hub row.
+-- 2. Ensure the parent physics-8 hub row exists.
+INSERT OR IGNORE INTO textbooks
+  (slug, subject, grade, title, author, description, html_path, para_count, color, sort_order, is_active)
+VALUES
+  ('physics-8', 'physics', 8, 'Физика — 8 класс',
+   '',
+   'Полный курс физики 8 класса: тепловые явления (§§1–11), электромагнитные явления (§§12–31), световые явления (§§32–40), 7 виртуальных лабораторных работ.',
+   'physics_8_hub.html', 47, 'violet', 4, 1);
+
+-- 3. Update the parent physics-8 hub row (covers case where it already existed from legacy migrations).
 UPDATE textbooks
 SET
   author      = '',