Pixmenia v20 Critical Scenarios Audit

Fixed:
- Strengthened admin auth/logging in pixboost_app/admin/persistent_assets.php
- Rebuilt persistent assets helper with health checks: includes/persistent_assets.php
- Rebuilt persistent assets helper with health checks: pixboost_app/includes/persistent_assets.php
- Added additive-only safe update guard SQL

PHP Syntax:
PASS

Warnings / must verify:
- Upload validation may be weak: pixboost_app/admin/settings.php
- Webhook idempotency needs live/code confirmation: pixboost_app/webhooks/paypal.php
- Webhook idempotency needs live/code confirmation: pixboost_app/webhooks/stripe.php

Most dangerous scenario: importing a full fresh database over production can delete customer records, credits, invoices, image paths, and legal approvals. Persistent Assets protects files, not overwritten database records.