fix(ansible): a daily schedule holds its time of day instead of creeping later (#5432)

Each firing stored last_run_at = the moment the ~60 s loop noticed it,
and the next was due a whole interval after that, so every firing's
lateness carried into the next: Nightly Docker Prune moved from 05:44:45
to 05:47:05 over six days.

A firing now stores its due slot, the latest last_run_at + k * interval
not after now (due_slot), for both a launch and a skipped firing. Missed
slots after an outage are skipped, not run in a burst.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_ansible_schedule_due.py

@@ -2,7 +2,7 @@
 from datetime import datetime, timedelta, timezone
 from types import SimpleNamespace
 
-from steward.ansible.scheduler import is_due
+from steward.ansible.scheduler import due_slot, is_due
 
 NOW = datetime(2026, 6, 16, 12, 0, 0, tzinfo=timezone.utc)
 
@@ -26,3 +26,27 @@ def test_due_when_interval_elapsed():
 
 def test_not_due_before_interval():
     assert is_due(_sched(last=NOW - timedelta(seconds=1800), interval=3600), NOW) is False
+
+
+def test_a_daily_schedule_holds_its_time_of_day():
+    """#5432: noticed 40 s late every day, a daily run still belongs to its
+    05:44:45 slot, so the lateness never adds up."""
+    day = 86400
+    last = datetime(2026, 10, 2, 5, 44, 45, tzinfo=timezone.utc)
+    for _ in range(6):
+        noticed = last + timedelta(seconds=day + 40)
+        assert is_due(_sched(last=last, interval=day), noticed)
+        last = due_slot(_sched(last=last, interval=day), noticed)
+    assert last == datetime(2026, 10, 8, 5, 44, 45, tzinfo=timezone.utc)
+
+
+def test_missed_slots_are_skipped_not_run_in_a_burst():
+    last = NOW - timedelta(hours=5, minutes=10)
+    slot = due_slot(_sched(last=last, interval=3600), NOW)
+    assert slot == NOW - timedelta(minutes=10)
+    assert not is_due(_sched(last=slot, interval=3600), NOW)
+
+
+def test_a_first_run_is_its_own_slot():
+    assert due_slot(_sched(last=None), NOW) == NOW
+