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/pages/SiteSettings.vue

@@ -135,9 +135,8 @@ const message = useMessage()
 const loading = ref(true)
 const saving = ref(false)
 
-// 基础设置（aksrt_basic_settings）
-// 后端 /settings/basic 已经做了部分合并，本接口只发送基础字段；
-// auth 相关字段由 /settings/auth 独立维护，互不影响。
+// 基础设置（canonical: aksrt_settings）
+// 后端 /settings/basic 返回基础字段；auth 相关字段由 /settings/auth 独立维护。
 const basic = reactive({
   comment_enabled: true,
   comment_requires_registration: false,