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.Connectors.Shared/ConnectorHashing.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace StellaOps.Notify.Connectors.Shared;
+
+/// <summary>
+/// Common hashing helpers for Notify connector metadata.
+/// </summary>
+public static class ConnectorHashing
+{
+    /// <summary>
+    /// Computes a lowercase hex SHA-256 hash and truncates it to the requested number of bytes.
+    /// </summary>
+    public static string ComputeSha256Hash(string value, int lengthBytes = 8)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            throw new ArgumentException("Value must not be null or whitespace.", nameof(value));
+        }
+
+        if (lengthBytes <= 0 || lengthBytes > 32)
+        {
+            throw new ArgumentOutOfRangeException(nameof(lengthBytes), "Length must be between 1 and 32 bytes.");
+        }
+
+        var bytes = Encoding.UTF8.GetBytes(value.Trim());
+        var hash = SHA256.HashData(bytes);
+        return Convert.ToHexString(hash.AsSpan(0, lengthBytes)).ToLowerInvariant();
+    }
+}