Extract IDP configuration to external JSON file

Move all hard-coded identity server settings (API resource, scope, client,
users/roles) from Program.cs into config/idp-config.json and add parsing
logic. The config file is copied to the output directory so it works for
local development and is available at /app/config/idp-config.json in the
Docker image, making it straightforward to override via a volume mount.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SimpleIdp.csproj

@@ -10,6 +10,11 @@
 		<PackageReference Include="SimpleIdServer.IdServer" Version="6.0.*-*" />
 		<PackageReference Include="Microsoft.Web.LibraryManager.Build" Version="2.1.175" />
 	</ItemGroup>
+	<ItemGroup>
+	  <Content Include="config\idp-config.json">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </Content>
+	</ItemGroup>
 	<ItemGroup>
 	  <Compile Update="Resources\AccountsResource.Designer.cs">
 	    <DependentUpon>AccountsResource.resx</DependentUpon>