Replace cleanup service datetime.utcnow calls (#1494)

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

tests/test_cleanup_service_utcnow.py

@@ -0,0 +1,25 @@
+"""Regression tests for the datetime.utcnow() removal in src/cleanup_service.py (#1116).
+
+Importing src.cleanup_service is cheap and dependency-free: its only module-level
+imports are logging/datetime/typing, and the `from src.database import ...` calls are
+lazy (inside the functions), so no DB/sqlalchemy stack is pulled in here.
+"""
+from datetime import datetime, timedelta, timezone
+
+from src.cleanup_service import _utcnow
+
+
+def test_utcnow_returns_naive_utc():
+    now = _utcnow()
+    # Must be naive to match the naive DateTime columns this module compares against.
+    assert now.tzinfo is None
+    ref = datetime.now(timezone.utc).replace(tzinfo=None)
+    assert abs((ref - now).total_seconds()) < 5
+
+
+def test_cutoff_math_stays_naive_and_comparable():
+    # Guards the archive/delete cutoffs against a naive/aware TypeError regression:
+    # an aware _utcnow() would raise when compared with the naive last_accessed column.
+    cutoff = _utcnow() - timedelta(days=7)
+    assert cutoff.tzinfo is None
+    assert cutoff < _utcnow()