mbproxy: close out the dashboard code-review minor findings

Resolves the remaining Minor items from the 2026-05-15 review so the
web-UI dashboard work has no open follow-ups: a real-HubConnection
end-to-end test for the SignalR feed, stable mbproxy.admin.broadcast.*
log-event names, keyboard/aria accessibility on the fleet table,
frontend JS hardening (URL-decode guard, NaN guards, shared util.js),
reconciler<->capture-registry coverage, throwing-sink and embedded-asset
tests, broadcaster polish, and a soft upper bound on AdminPushIntervalMs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Options/MbproxyOptions.cs

@@ -11,7 +11,8 @@ public sealed class MbproxyOptions
     /// <summary>
     /// Server-push cadence (milliseconds) for the admin dashboard's SignalR feed.
     /// Every interval the admin endpoint builds a status snapshot and pushes it to
-    /// connected dashboard / detail-page clients. Must be &gt; 0. Defaults to 1000.
+    /// connected dashboard / detail-page clients. Must be in the range 1–60000 ms
+    /// (a value past a minute makes the "live" feed non-live). Defaults to 1000.
     /// </summary>
     public int AdminPushIntervalMs { get; init; } = 1000;
 
@@ -114,8 +115,12 @@ public sealed class MbproxyOptionsValidator : IValidateOptions<MbproxyOptions>
             errors.Add(
                 $"Connection.GracefulShutdownTimeoutMs must be > 0; got {options.Connection.GracefulShutdownTimeoutMs}.");
 
-        if (options.AdminPushIntervalMs <= 0)
-            errors.Add($"AdminPushIntervalMs must be > 0; got {options.AdminPushIntervalMs}.");
+        // AdminPushIntervalMs has a soft upper bound: a value past a minute makes the
+        // dashboard's "live" feed effectively non-live, which is almost always a typo
+        // (e.g. a seconds value pasted as milliseconds) rather than an intent.
+        if (options.AdminPushIntervalMs <= 0 || options.AdminPushIntervalMs > 60_000)
+            errors.Add(
+                $"AdminPushIntervalMs must be between 1 and 60000 ms; got {options.AdminPushIntervalMs}.");
 
         // Keepalive section ranges. Cross-field rules (heartbeat interval vs request
         // timeout) are enforced in ReloadValidator.