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.

inc/classes/class-aksrt-user-home.php

@@ -279,22 +279,24 @@ class AKSRT_User_Home {
     }
 
     /**
-     * @param array{source:string,thresholds:array<int,int>} $level_settings
+     * @param array{source?:string,thresholds?:array<int,int>} $level_settings
      */
     private function get_user_level(int $user_id, array $level_settings): int {
-        if (($level_settings['source'] ?? 'credits_balance') === 'manual_meta') {
+        $source = (string) ($level_settings['source'] ?? 'integral');
+        if ($source === 'manual_meta') {
             return max(0, (int) get_user_meta($user_id, 'aksrt_user_level', true));
         }
-
+        if (class_exists('AKSRT_User_Level')) {
+            return AKSRT_User_Level::get_level($user_id);
+        }
+        // Fallback: credits_balance thresholds (legacy) if integral class missing.
         $score = function_exists('aksrt_user_credits') ? (int) aksrt_user_credits($user_id) : 0;
         $level = 0;
-
         foreach ((array) ($level_settings['thresholds'] ?? []) as $candidate => $threshold) {
             if ($score >= (int) $threshold) {
                 $level = (int) $candidate;
             }
         }
-
         return $level;
     }