hardening: migrate quicktree SQL helpers to prepared variants

[somethingwithproof]

Problem

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

Proposed change

• migrate straightforward read queries to prepared helper variants
• preserve existing plugin behavior
• keep scope to safe mechanical conversions

Tests

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