hardening: migrate remaining DB helpers to prepared statements

[somethingwithproof]

Problem\nSeveral plugin DB helper calls still use non-prepared variants.\n\n## Proposed change\n- migrate remaining db_execute/db_fetch_* calls in core plugin files to *_prepared variants\n- keep SQL behavior unchanged\n\n## Tests\n- add a regression script that fails if raw DB helper variants remain in targeted files\n- run existing standalone plugin tests\n