refactor(settings): collapse get_all to aksrt_settings and drop migration UI

Make aksrt_settings the sole theme settings store for get_all(), remove
aksrt_seo_settings/aksrt_basic_settings merge paths, point global.logo
schema at aksrt_settings, and delete the retired settings migration page,
route, REST endpoint, and related SPA surface.

assets/admin/src/schema/sections/tools/migration.js

@@ -1,19 +0,0 @@
-export const migrationSchema = {
-  key: 'tools.migration',
-  title: '数据迁移',
-  icon: 'fas fa-database',
-  description: '数据库结构迁移和版本升级。',
-  defaults: {},
-  groups: [
-    {
-      key: 'status',
-      title: '迁移状态',
-      icon: 'fas fa-info-circle',
-      fields: [
-        { key: 'current_version', label: '当前版本', type: 'display' },
-        { key: 'pending_migrations', label: '待执行迁移', type: 'display' },
-        { key: 'run_migration', label: '执行迁移', type: 'button', help: '手动触发数据库迁移' },
-      ],
-    },
-  ],
-}