Add tests and implement StubBearer authentication for Signer endpoints

- Created SignerEndpointsTests to validate the SignDsse and VerifyReferrers endpoints.
- Implemented StubBearerAuthenticationDefaults and StubBearerAuthenticationHandler for token-based authentication.
- Developed ConcelierExporterClient for managing Trivy DB settings and export operations.
- Added TrivyDbSettingsPageComponent for UI interactions with Trivy DB settings, including form handling and export triggering.
- Implemented styles and HTML structure for Trivy DB settings page.
- Created NotifySmokeCheck tool for validating Redis event streams and Notify deliveries.

src/StellaOps.Notify.Engine/ChannelHealthContracts.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using StellaOps.Notify.Models;
+
+namespace StellaOps.Notify.Engine;
+
+/// <summary>
+/// Contract implemented by channel plug-ins to provide health diagnostics.
+/// </summary>
+public interface INotifyChannelHealthProvider
+{
+    /// <summary>
+    /// Channel type supported by the provider.
+    /// </summary>
+    NotifyChannelType ChannelType { get; }
+
+    /// <summary>
+    /// Executes a health check for the supplied channel.
+    /// </summary>
+    Task<ChannelHealthResult> CheckAsync(ChannelHealthContext context, CancellationToken cancellationToken);
+}
+
+/// <summary>
+/// Immutable context describing a channel health request.
+/// </summary>
+public sealed record ChannelHealthContext(
+    string TenantId,
+    NotifyChannel Channel,
+    string Target,
+    DateTimeOffset Timestamp,
+    string TraceId);
+
+/// <summary>
+/// Result returned by channel plug-ins when reporting health diagnostics.
+/// </summary>
+public sealed record ChannelHealthResult(
+    ChannelHealthStatus Status,
+    string? Message,
+    IReadOnlyDictionary<string, string> Metadata);
+
+/// <summary>
+/// Supported channel health states.
+/// </summary>
+public enum ChannelHealthStatus
+{
+    Healthy,
+    Degraded,
+    Unhealthy
+}