feat(users): level integral XP schema + AKSRT_User_Level runtime

Rewrite /users/level to Zibll-like user_level_s/max/opt and
user_integral_opt. Runtime grants XP with day_max, resolves level,
hooks login/register/post/comment/like/favorite/follow. Profile
signals read integral-backed levels. audit errors=0.

functions.php

@@ -370,6 +370,9 @@ if (class_exists('AKSRT_User_Badges')) {
 if (class_exists('AKSRT_User_Badges_Admin')) {
     AKSRT_User_Badges_Admin::get_instance()->init();
 }
+if (class_exists('AKSRT_User_Level')) {
+    AKSRT_User_Level::get_instance()->init();
+}
 
 // WordPress 原生侧栏：支付/订单运营中心（zibll zibpay 对齐）
 if (is_admin() && class_exists('AKSRT_Admin_Pay_Menus')) {