fix(alerts): Steward's own outage no longer reads as every agent going silent (#5441)

After Steward or its database was down, every agent's age and every
series' last sample were old, so all agent-offline rules fired and all
rules went no-data at once, clearing a minute later. With per-account
alert email that would have mailed everyone a dozen notices per restart.

A periodic job now ticks a Watch (steward/core/watch.py) once it has
reached the database; the Watch says since when runs have been unbroken
(process start, or the end of a gap over 120 s). seconds_since_seen and
the no-data sweep count silence from no earlier than that, so a source
must stay quiet while Steward is watching before it counts as silent.

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

tests/core/test_alert_no_data.py

@@ -60,6 +60,19 @@ def test_silent_since(last_ago, interval, watch_ago, silent_after):
         assert got == (last or watch) + timedelta(seconds=silent_after)
 
 
+def test_steward_s_own_outage_is_not_the_series_silence():
+    """#5441: a series last sampled 18 minutes ago, while Steward has only been
+    back for 2 minutes, is not silent yet; once Steward has watched it for
+    three of its gaps with nothing arriving, it is, from then."""
+    last = NOW - timedelta(minutes=18)
+    watch = NOW - timedelta(days=1)
+    assert silent_since(last, 60, watch, NOW, awake_since=NOW - timedelta(minutes=2)) is None
+    awake = NOW - timedelta(minutes=6)
+    assert silent_since(last, 60, watch, NOW, awake_since=awake) == awake + timedelta(seconds=300)
+    # A sample since Steward came back is what counts, when there is one.
+    assert silent_since(NOW - timedelta(minutes=1), 60, watch, NOW, awake_since=awake) is None
+
+
 class _Session:
     def __init__(self):
         self.added = []