WEB-2: Self-hosted public site (publicsite-nginx + POST /api/publish, atomic swap)

POST /api/publish: rendert SiteSnapshot->HTML in _staging_new/, atomic swap ->
live/ (rename, ein Syscall). publicsite-nginx:alpine serviert live/ read-only
auf Port 8081. Shared Volume api(rw)/publicsite(ro). Manager bleibt LAN-only.
5 neue Tests (atomic swap, UTF-8, mehrfach), 184/184 gruen. compose config OK.
Vhost-Snippet + web-deploy.md (Deutsch) beigelegt; <DOMAIN> wartet auf Julian.

GerbilManagerWebAPI/Cms/PublicSiteOptions.cs

@@ -0,0 +1,16 @@
+namespace GerbilManagerWebAPI.Cms
+{
+    /// <summary>
+    /// WEB-2: path where POST /api/publish writes the rendered static site.
+    /// Env var: PublicSite__RootPath (empty = publish disabled, returns 503).
+    /// In production this volume is shared with the publicsite-nginx container.
+    /// </summary>
+    public sealed class PublicSiteOptions
+    {
+        public const string SectionName = "PublicSite";
+
+        public string? RootPath { get; set; }
+
+        public bool IsConfigured => !string.IsNullOrWhiteSpace(RootPath);
+    }
+}