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.Excititor.Core/VexConsensusHold.cs

@@ -0,0 +1,47 @@
+namespace StellaOps.Excititor.Core;
+
+public sealed record VexConsensusHold
+{
+    public VexConsensusHold(
+        string vulnerabilityId,
+        string productKey,
+        VexConsensus candidate,
+        DateTimeOffset requestedAt,
+        DateTimeOffset eligibleAt,
+        string reason)
+    {
+        if (string.IsNullOrWhiteSpace(vulnerabilityId))
+        {
+            throw new ArgumentException("Vulnerability id must be provided.", nameof(vulnerabilityId));
+        }
+
+        if (string.IsNullOrWhiteSpace(productKey))
+        {
+            throw new ArgumentException("Product key must be provided.", nameof(productKey));
+        }
+
+        if (eligibleAt < requestedAt)
+        {
+            throw new ArgumentOutOfRangeException(nameof(eligibleAt), "EligibleAt cannot be earlier than RequestedAt.");
+        }
+
+        VulnerabilityId = vulnerabilityId.Trim();
+        ProductKey = productKey.Trim();
+        Candidate = candidate ?? throw new ArgumentNullException(nameof(candidate));
+        RequestedAt = requestedAt;
+        EligibleAt = eligibleAt;
+        Reason = string.IsNullOrWhiteSpace(reason) ? "unspecified" : reason.Trim();
+    }
+
+    public string VulnerabilityId { get; }
+
+    public string ProductKey { get; }
+
+    public VexConsensus Candidate { get; }
+
+    public DateTimeOffset RequestedAt { get; }
+
+    public DateTimeOffset EligibleAt { get; }
+
+    public string Reason { get; }
+}