fix(traefik): a router fails on 5xx only once it answered a few server errors (#5865)

One 502 among about 70 requests read steward@swarm critical at 1.4%:
past the 30-request minimum and far past the 0.1% cutoff, yet one
error. A new Settings → Thresholds field, "Fewest server errors to
fail" (traefik.threshold.min_5xx, default 3), must also be reached in
the 15-minute window; the 0.1% cutoff still catches a busy router.

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

tests/plugins/traefik/test_normals.py

@@ -74,6 +74,14 @@ def test_a_share_over_a_handful_of_requests_is_not_graded():
     assert router_state(_window(e5=12.5, requests=30.0), TH, "")[0] == "crit"
 
 
+def test_a_5xx_share_fails_only_once_enough_errors_were_answered():
+    # steward@swarm on 2026-10-15: one 502 among about 70 requests (#5865)
+    assert router_state(_window(e5=1.43, requests=70.0), TH, "") == ("ok", None)
+    assert router_state(_window(e5=4.29, requests=70.0), TH, "")[0] == "crit"      # 3 errors
+    stricter = to_thresholds_cfg({"traefik.threshold.min_5xx": 1})
+    assert router_state(_window(e5=1.43, requests=70.0), stricter, "")[0] == "crit"
+
+
 def test_grade_routers_hands_each_router_its_own_normals():
     routers = [{"name": "errors@docker", "latest": _latest(e4=60.0)},
                {"name": "app@docker", "latest": _latest(e4=60.0)}]