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/settings/routes.py

@@ -146,6 +146,7 @@ _THRESHOLD_FIELDS = [
     ("error_5xx_pct", "traefik.threshold.error_5xx_pct", True),
     ("error_4xx_pct", "traefik.threshold.error_4xx_pct", True),
     ("min_requests", "traefik.threshold.min_requests", False),
+    ("min_5xx", "traefik.threshold.min_5xx", False),
     # Each normal's floor (#5742), named by its own setting key.
     *[(f"norms.floor.{name}", f"norms.floor.{name}", True) for name in NORM_FLOORS],
 ]