feat(oauth): expose RFC 9728 protected resource metadata endpoint

Claude.ai's MCP OAuth client only sends the RFC 8707 `resource` param
when it has Protected Resource Metadata (PRM) to learn the resource
identifier from. Without it, nas-auth's /authorize rejected the request
with 'resource 必填'.

Adds /.well-known/oauth-protected-resource (RFC 9728) that returns the
resource URL + AS URL + supported scopes. ResourceUrl is configurable
via Mcp__OAuthDiscovery__ResourceUrl, falling back to request authority
when unset (works in local dev).

Config/McpDiscoveryOptions.cs

@@ -1,8 +1,8 @@
 namespace ObsidianMcp.Config;
 
 /// <summary>
-/// /.well-known/oauth-authorization-server 返回的元数据配置。
-/// 环境变量前缀 Mcp__OAuthDiscovery__，例如 Mcp__OAuthDiscovery__Issuer=...
+/// /.well-known/oauth-authorization-server + /.well-known/oauth-protected-resource
+/// 端点返回的元数据。环境变量前缀 Mcp__OAuthDiscovery__。
 /// </summary>
 public class McpDiscoveryOptions
 {
@@ -12,4 +12,11 @@ public class McpDiscoveryOptions
     public string AuthorizationEndpoint { get; set; } = "https://auth.zhengchentao.win/authorize";
     public string TokenEndpoint { get; set; } = "https://auth.zhengchentao.win/token";
     public string RegistrationEndpoint { get; set; } = "https://auth.zhengchentao.win/register";
+
+    /// <summary>
+    /// 本资源服务的标识符（RFC 9728 PRM 的 `resource` 字段，必须与 nas-auth
+    /// resources.json 里 obsidian 条目的 resource_url 完全一致）。
+    /// 留空时 PRM 端点回退用请求的 `scheme://host`。
+    /// </summary>
+    public string ResourceUrl { get; set; } = string.Empty;
 }