feat(traefik): a 4xx share is worth a look only once it is a lot of 4xx (#6105)

Three 404s on a quiet router read 7.5% and were raised as an anomaly; most of
the 4xx anomalies on review stood for 3 to 7 errors in 15 minutes. A new
Settings → Thresholds minimum, traefik.threshold.min_4xx (default 20, more
than one a minute held for the window), applies to both the router's grade
on its page and to anomaly detection, as min_5xx does for 5xx.

learn_normal takes `fewest`: for a share gated on a per-second rate, the
fewest events (rate × window × share) it must stand for to be judged.

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

tests/core/test_anomalies.py

@@ -9,7 +9,7 @@ from zoneinfo import ZoneInfo
 from quart import render_template, session
 
 from steward.core import mail
-from steward.core.anomalies import DEPART, AnomalyView, departs
+from steward.core.anomalies import DEPART, WINDOW, AnomalyView, _gate_open, departs
 from steward.core.norms import Norm
 from tests.test_style_tokens import hex_colours
 
@@ -146,3 +146,15 @@ def test_a_week_without_anomalies_leaves_the_section_out():
     quiet = {**REPORT, "anomalies": {"seen": [], "lapsed": [], "to_review": 0}}
     _, text, html = mail.report_email({}, quiet, to)
     assert "anomal" not in (text + html).lower()
+
+
+def test_a_share_gated_on_a_rate_is_judged_only_once_it_stands_for_enough_events():
+    """#6105: three 404s on a quiet router read 7%. With `fewest`, the share
+    is judged only once rate × window × share reaches it."""
+    learned = SimpleNamespace(gate=("req", lambda: 30 / WINDOW.total_seconds()), fewest=lambda: 20)
+    key = ("traefik", "r", "e4")
+    per_s = 100 / WINDOW.total_seconds()                               # 100 requests in the window
+    assert not _gate_open(learned, key, {key: 7.5, ("traefik", "r", "req"): per_s})          # 7.5 errors
+    assert _gate_open(learned, key, {key: 25.0, ("traefik", "r", "req"): per_s})             # 25
+    assert not _gate_open(learned, key, {key: 90.0, ("traefik", "r", "req"): 10 / WINDOW.total_seconds()})
+    assert _gate_open(SimpleNamespace(gate=("req", 0.0), fewest=None), key, {key: 1.0})