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).

Endpoints/DiscoveryEndpoints.cs

@@ -5,9 +5,11 @@ namespace ObsidianMcp.Endpoints;
 public static class DiscoveryEndpoints
 {
     /// <summary>
-    /// 注册 /.well-known/oauth-authorization-server 端点。
-    /// 返回静态 JSON 指向 nas-auth，让 Claude.ai 能自动发现授权服务器。
-    /// 所有字段从 McpDiscoveryOptions 读，不 hardcode。
+    /// 注册两个 well-known 端点：
+    /// 1. /.well-known/oauth-authorization-server (RFC 8414)：指向 nas-auth
+    /// 2. /.well-known/oauth-protected-resource (RFC 9728)：告诉客户端这个资源
+    ///    的 identifier 是什么 + 用哪个 AS。Claude.ai 读到 PRM 后会在
+    ///    /authorize 请求里带 resource=<identifier>，满足 nas-auth 的 RFC 8707 校验。
     /// </summary>
     public static void MapDiscoveryEndpoints(this WebApplication app)
     {
@@ -25,7 +27,23 @@ public static class DiscoveryEndpoints
                 scopes_supported = new[] { "read:obsidian", "write:obsidian" },
             });
         })
-        .AllowAnonymous() // well-known 端点不需要认证，Claude 在登录前就要访问它
+        .AllowAnonymous()
         .WithName("OAuthDiscovery");
+
+        app.MapGet("/.well-known/oauth-protected-resource", (HttpContext ctx, McpDiscoveryOptions opts) =>
+        {
+            var resourceUrl = string.IsNullOrWhiteSpace(opts.ResourceUrl)
+                ? $"{ctx.Request.Scheme}://{ctx.Request.Host}"
+                : opts.ResourceUrl;
+            return Results.Ok(new
+            {
+                resource = resourceUrl,
+                authorization_servers = new[] { opts.Issuer },
+                scopes_supported = new[] { "read:obsidian", "write:obsidian" },
+                bearer_methods_supported = new[] { "header" },
+            });
+        })
+        .AllowAnonymous()
+        .WithName("ProtectedResourceMetadata");
     }
 }