Split Program.cs DI registrations into per-namespace DependencyRegistration classes

Each root namespace (Audit, Common, Data, Environments, Features/Usage,
Identities, Organizations, Projects, Segments, Users, Webhooks) gets its
own static DependencyRegistration class exposing an Add<Name>Services
extension method, so Program.cs just wires them together instead of
listing every registration inline.

src/api/MicCheck.Api/Webhooks/DependencyRegistration.cs

@@ -0,0 +1,17 @@
+namespace MicCheck.Api.Webhooks;
+
+public static class DependencyRegistration
+{
+    public static IServiceCollection AddWebhooksServices(this IServiceCollection services)
+    {
+        services.AddScoped<WebhookService>();
+        services.AddScoped<WebhookDispatcher>();
+        services.AddSingleton<WebhookQueue>();
+        services.AddHostedService<WebhookBackgroundService>();
+        services.AddHostedService<WebhookRetryBackgroundService>();
+        services.AddHttpClient("Webhooks", client =>
+            client.DefaultRequestHeaders.Add("User-Agent", "MicCheck-Webhook/1.0"));
+
+        return services;
+    }
+}