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/templates/settings/thresholds.html

@@ -86,6 +86,12 @@
     <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">Below this, a router's 4xx and 5xx shares aren't judged, on its page or as an anomaly: one 404 of three requests is 33%, two 500s of sixteen 12%.
       A quiet router that fails every request is left to its uptime monitor until it has this many.</div>
   </div>
+  <div class="form-group" style="margin:1.1rem 0 0.25rem;">
+    <label for="min_5xx">Fewest server errors to fail <span style="color:var(--text-muted);font-size:0.8rem;">(5xx in 15 minutes)</span></label>
+    <input type="number" id="min_5xx" name="min_5xx" min="1" step="1"
+           value="{{ settings['traefik.threshold.min_5xx'] }}" style="max-width:110px;margin-top:0.25rem;">
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">A router past the 5xx share above is failing only once it has answered this many 5xx: one 502 among 70 requests is 1.4%, but it is one error.</div>
+  </div>
 </div>
 {% endif %}