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/settings/class-aksrt-settings-user-runtime.php

@@ -149,11 +149,12 @@ class AKSRT_Settings_User_Runtime {
                 'enabled'        => !empty($settings['user_level_enabled']) || !empty($settings['user_level_s']),
                 'public_display' => !isset($settings['user_level_public_display']) || !empty($settings['user_level_public_display']),
                 'source'         => (static function () use ($settings) {
-                    $v = $settings['user_level_source'] ?? 'credits_balance';
-                    return in_array($v, ['credits_balance', 'manual_meta', 'integral'], true) ? $v : 'credits_balance';
+                    $v = $settings['user_level_source'] ?? 'integral';
+                    return in_array($v, ['credits_balance', 'manual_meta', 'integral'], true) ? $v : 'integral';
                 })(),
                 'label_prefix'   => $level_prefix !== '' ? $level_prefix : 'Lv.',
                 'thresholds'     => self::parse_level_thresholds((string) ($settings['user_level_thresholds'] ?? "1|0\n2|30\n3|100\n4|300")),
+                'max'            => max(3, min(30, (int) ($settings['user_level_max'] ?? 10))),
             ],
             'medal' => $medal,
             // Alias for existing call sites that still read badge.*; dictionary is derived from medal catalog.