ARCH-2: add Aspire AppHost + ServiceDefaults (Aspire 13.4.2)

- New projects GerbilManager.AppHost / GerbilManager.ServiceDefaults (templates)
- Added both to GerbilManager.slnx
- WebAPI references ServiceDefaults; AddServiceDefaults + MapDefaultEndpoints wired

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GerbilManager.AppHost/aspire.config.json

@@ -0,0 +1,5 @@
+{
+  "appHost": {
+    "path": "GerbilManager.AppHost.csproj"
+  }
+}