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/Properties/launchSettings.json

@@ -0,0 +1,29 @@
+{
+  "$schema": "https://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "https": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": true,
+      "applicationUrl": "https://localhost:17004;http://localhost:15265",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development",
+        "DOTNET_ENVIRONMENT": "Development",
+        "ASPIRE_DASHBOARD_OTLP_ENDPOINT_URL": "https://localhost:21284",
+        "ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL": "https://localhost:22112"
+      }
+    },
+    "http": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": true,
+      "applicationUrl": "http://localhost:15265",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development",
+        "DOTNET_ENVIRONMENT": "Development",
+        "ASPIRE_DASHBOARD_OTLP_ENDPOINT_URL": "http://localhost:19171",
+        "ASPIRE_RESOURCE_SERVICE_ENDPOINT_URL": "http://localhost:20259"
+      }
+    }
+  }
+}