hardening: migrate audit SQL helpers to prepared variants

[somethingwithproof]

Problem

plugin_audit still has raw SQL helper usage (db_fetch_cell, db_fetch_assoc, db_execute) in setup/runtime paths.

Proposed change

• migrate straightforward call sites to prepared helper variants
• parameterize values currently interpolated into SQL where applicable
• preserve existing plugin behavior

Tests

• add regression checks for converted prepared helper usage in touched files
• run syntax checks for modified files and test script

[somethingwithproof]

Related: Grok review (2026-04-08) confirmed unserialize at audit_functions.php:151 as TRUE POSITIVE (MEDIUM). The allowed_classes=false mitigates but recommend migrating to sanitize_unserialize_selected_items().

[somethingwithproof]

Fix PR open: #52