hardening: migrate gexport SQL helpers to prepared variants

[somethingwithproof]

Problem

plugin_gexport still has several raw SQL helper call sites (db_fetch_cell, db_fetch_assoc, db_execute) in runtime/setup 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