fix(traefik): a router's 5xx share is judged only over enough requests, like its 4xx share (#5859)

Two 500s of about sixteen requests graded roundtable@swarm critical at
9.1%. The Settings → Thresholds minimum of requests in 15 minutes (30 by
default) now guards both shares; the 5xx share still keeps its fixed
cutoff whatever the router's history.

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

steward/templates/settings/thresholds.html

@@ -80,10 +80,11 @@
       so one that always answers some 4xx stops being listed.</div>
   </div>
   <div class="form-group" style="margin:1.1rem 0 0.25rem;">
-    <label for="min_requests">Fewest requests to judge 4xx <span style="color:var(--text-muted);font-size:0.8rem;">(in 15 minutes)</span></label>
+    <label for="min_requests">Fewest requests to judge 4xx and 5xx <span style="color:var(--text-muted);font-size:0.8rem;">(in 15 minutes)</span></label>
     <input type="number" id="min_requests" name="min_requests" min="0" step="1"
            value="{{ settings['traefik.threshold.min_requests'] }}" style="max-width:110px;margin-top:0.25rem;">
-    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">Below this, a router's 4xx share isn't judged, on its page or as an anomaly: one 404 of three requests is 33%. 5xx is always judged.</div>
+    <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>
 {% endif %}