hardening: migrate apcupsd SQL helpers to prepared variants

[somethingwithproof]

Problem

plugin_apcupsd still uses some raw DB helpers (db_fetch_assoc / db_fetch_cell) at call sites that can be cleanly migrated to prepared variants.

Proposed change

• migrate straightforward SQL helper call sites to prepared variants
• preserve query behavior and plugin workflow
• avoid broad behavioral changes and focus on safe mechanical conversions

Tests

• add regression script asserting targeted files use prepared helper variants at converted call sites
• run syntax checks for modified files and test script