Pixmenia v23 Clean Admin Settings + Assets Protection

- Removed misplaced helper functions from pixboost_app/admin/database_guard.php
- Removed settings insert from sql/v19_persistent_assets.sql

PHP Syntax: PASS
SQL Safety: PASS

Admin/settings protected: logo, favicon, Stripe, PayPal, Resend, SEO, colors, packages.
Image repair: /admin/post_update_repair.php uses Persistent Assets to reconnect customer files after phpMyAdmin update.