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>

steward/core/settings.py

@@ -100,6 +100,9 @@ DEFAULTS: dict[str, Any] = {
     # A router's 4xx share over fewer requests than this in 15 minutes isn't
     # graded, on the page or for anomalies: one 404 of three is 33% (#5741).
     "traefik.threshold.min_requests": 30,
+    # ...and a 5xx share past its cutoff is failing only once this many 5xx
+    # were answered in those 15 minutes: one 502 of 70 is 1.4% (#5865).
+    "traefik.threshold.min_5xx": 3,
     # The smallest departure from a metric's normal worth calling an anomaly
     # (#5742), on top of being a quarter over its usual hold: a share's floor
     # is in percentage points, a response time's in ms. 5 points is the
@@ -553,6 +556,9 @@ def to_thresholds_cfg(settings: dict[str, Any]) -> dict:
         # Fewer requests than this in 15 minutes and a router's 4xx share isn't graded.
         "router_requests": {"warn": g("traefik.threshold.min_requests", 30),
                             "crit": g("traefik.threshold.min_requests", 30), "dir": "low"},
+        # Fewer 5xx than this in 15 minutes and a router isn't failing, whatever the share.
+        "router_5xx": {"warn": g("traefik.threshold.min_5xx", 3),
+                       "crit": g("traefik.threshold.min_5xx", 3), "dir": "low"},
         # Not a cutoff: each normal's floor (norms.floor.*), read by floor_of().
         "floors": {name: float(g(f"norms.floor.{name}", DEFAULTS[f"norms.floor.{name}"]))
                    for name in NORM_FLOORS},