Initial WebhookServer implementation

Add the .NET 8 solution scaffolded against PLAN.md. Three projects share
WebhookServer.Core (models, auth, execution, storage, IPC, callbacks)
and WebhookServer.Service hosts an embedded Kestrel listener plus the
named-pipe admin server. WebhookServer.Gui is a thin MVVM client over
the pipe. Includes 25 unit tests covering HMAC verification, bearer
auth, IP allowlist parsing, arg-template rendering, DPAPI round-trip,
and the encrypt-on-save config store.

Install/uninstall PowerShell scripts default to LocalSystem and accept
a domain user or gMSA via -ServiceAccount.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Core/Models/ServerConfig.cs

@@ -0,0 +1,17 @@
+namespace WebhookServer.Core.Models;
+
+public sealed class ServerConfig
+{
+    public int HttpPort { get; set; } = 8080;
+    public HttpsBinding? HttpsBinding { get; set; }
+
+    /// <summary>
+    /// IPs/CIDRs allowed to set X-Forwarded-For. Empty = forwarded headers are ignored
+    /// and the direct connection IP is always used.
+    /// </summary>
+    public List<string> TrustedProxies { get; set; } = new();
+
+    public int LogRetentionDays { get; set; } = 14;
+
+    public List<EndpointConfig> Endpoints { get; set; } = new();
+}