hardening: migrate quicktree SQL helpers to prepared variants #11
Description
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