refactor: one wpdb accessor style (L4)

The three lazy db() methods use the $this->db ?? $GLOBALS['wpdb'] one-liner that
SampledMetaSource, MediaUsageFinder and ConnectionRepository already use.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Integrations/Divi/Pages/DiviReplaceTextAbility.php

@@ -490,11 +490,6 @@ final class DiviReplaceTextAbility implements Ability
 
     private function db(): \wpdb
     {
-        if ($this->db === null) {
-            global $wpdb;
-            $this->db = $wpdb;
-        }
-
-        return $this->db;
+        return $this->db ?? $GLOBALS['wpdb'];
     }
 }