Pixmenia v21 Database Overwrite Guard

- Added includes/database_guard.php
- Added pixboost_app/includes/database_guard.php
- Added pixboost_app/admin/database_guard.php
- Added sql/v21_database_overwrite_guard.sql
- Added warning header to pixboost_app/sql/database.sql

PHP Syntax: PASS

Protected tables: users, customers, images, generated_images, user_images, enhancements, credit_transactions, payments, orders, invoices, legal_consents.

Important truth: PHP cannot stop a destructive phpMyAdmin import done manually with full database privileges. The protection works by forcing updates through Admin > Database Overwrite Guard and by making database.sql carry a warning header.