hardening: migrate maint SQL helpers to prepared variants #47
Description
Problem
plugin_maint still contains raw db_fetch_assoc helper usage in schedule and filter-list paths.
Proposed change
- migrate straightforward read queries to prepared helper variants
- preserve query semantics and output behavior
- keep scope focused on safe, mechanical conversions
Tests
- add regression checks for converted prepared helper usage in touched files
- run syntax checks for modified files and new test script