feat(infra): implement RefreshTokenGenerator with cryptographic random bytes

src/api/SIGCM2.Infrastructure/Security/RefreshTokenGenerator.cs

@@ -0,0 +1,17 @@
+using System.Security.Cryptography;
+using SIGCM2.Application.Abstractions.Security;
+
+namespace SIGCM2.Infrastructure.Security;
+
+public sealed class RefreshTokenGenerator : IRefreshTokenGenerator
+{
+    public string Generate()
+    {
+        Span<byte> bytes = stackalloc byte[32];
+        RandomNumberGenerator.Fill(bytes);
+        return Convert.ToBase64String(bytes)
+                      .TrimEnd('=')
+                      .Replace('+', '-')
+                      .Replace('/', '_');
+    }
+}