feat(dashboard): distinct 'forced' subtag provider badge

Render Fallback:Mode=ForceSubtag as a cyan 'Subtag monitoring (forced)'
badge, distinct from the amber failover 'degraded' badge, so an intentional
configuration isn't shown as a fault. Distinguished by the shared
AlarmProviderReasons.ForcedSubtag reason carried on the provider-status feed.

src/ZB.MOM.WW.MxGateway.Server/Alarms/AlarmProviderReasons.cs

@@ -0,0 +1,18 @@
+namespace ZB.MOM.WW.MxGateway.Server.Alarms;
+
+/// <summary>
+///     Well-known <c>reason</c> strings carried on the alarm feed's
+///     <c>AlarmProviderStatus</c> message. Shared between the producer
+///     (<see cref="GatewayAlarmMonitor" />) and consumers (e.g. the dashboard
+///     provider badge) so the two cannot drift on a magic string.
+/// </summary>
+public static class AlarmProviderReasons
+{
+    /// <summary>
+    ///     Reason set when the monitor starts in subtag mode because
+    ///     <c>MxGateway:Alarms:Fallback:Mode</c> is <c>ForceSubtag</c> — a
+    ///     deliberate configuration, not a runtime failover. Lets the dashboard
+    ///     distinguish a forced subtag mode from an unexpected degraded failover.
+    /// </summary>
+    public const string ForcedSubtag = "Forced subtag mode (configuration)";
+}

src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs

@@ -170,7 +170,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             case AlarmProviderMode.Subtag:
                 initialMode = AlarmProviderMode.Subtag;
                 initialDegraded = true;
-                initialReason = "Forced subtag mode (configuration)";
+                initialReason = AlarmProviderReasons.ForcedSubtag;
                 break;
             case AlarmProviderMode.Alarmmgr:
                 initialMode = AlarmProviderMode.Alarmmgr;