datetime.utcnow() is deprecated in Python 3.12 and removed in 3.14.
Swap the five calls in src/cleanup_service.py for a local _utcnow()
helper returning naive UTC, matching the naive DateTime columns the
archive/delete cutoffs compare against (same approach as the
task-scheduler and core-database slices). Add a regression test
asserting the helper stays naive so the cutoff math can't hit a
naive/aware TypeError.
Part of #1116
