feat(modules): module topics for student form, admin CRUD with pagination

- db: modules table (name, lessons_count) + entries.module_id (ON DELETE SET NULL), migration + idempotent startup ensure
- api: GET /api/modules (public, search + limit/offset, entries_count), POST/PUT/DELETE (admin, audit-logged)
- entries: accept/validate module_id on create/update, return module_name, module_id filter
- backup/restore: include modules and entries.module_id
- student form: required module select, hidden while no modules exist
- admin: modules.html + js/modules.js list with pagination, search, create/edit/delete modal
- journal: module filter, module select in edit modal, module badge, CSV column

public/admin.js

@@ -56,6 +56,7 @@ function buildSidebar(active) {
     { page: 'trash', label: 'Корзина', icon: 'trash-2' },
   ];
   const adminNav = [
+    { page: 'modules', label: 'Темы модулей', icon: 'layers' },
     { page: 'branches', label: 'Филиалы', icon: 'building-2' },
     { page: 'users', label: 'Пользователи', icon: 'user-cog' },
     { page: 'worker', label: 'Воркер ИИ', icon: 'bot' },