feat(app): add IClientContext abstraction for IP and UserAgent

src/api/SIGCM2.Application/Abstractions/IClientContext.cs

@@ -0,0 +1,12 @@
+namespace SIGCM2.Application.Abstractions;
+
+/// <summary>
+/// Provides HTTP client metadata (IP address and User-Agent) from the current request context.
+/// Implemented in Infrastructure via IHttpContextAccessor.
+/// Mockable in tests without HTTP stack.
+/// </summary>
+public interface IClientContext
+{
+    string Ip { get; }
+    string? UserAgent { get; }
+}