Try to fix boardcasting issue.

Server/Security/AuthHelper.cs

@@ -3,7 +3,10 @@ using System.Security.Claims;
 using System.IdentityModel.Tokens.Jwt;
 using Microsoft.AspNetCore.Http;
 using Microsoft.IdentityModel.Tokens;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
 using SNote.Server.Security;
+using SNote.Server.Endpoints;
 
 namespace SNote.Server.Security;
 
@@ -47,7 +50,7 @@ public static class AuthHelper
 
     public static bool IsServerTokenValid(HttpContext context, PeerCache peerCache)
     {
-        var serverUrl = context.Request.Headers["X-Server-Url"].ToString();
+        var serverUrl = context.Request.Headers["X-Server-Url"].ToString().Trim().TrimEnd('/');
         var serverToken = context.Request.Headers["X-Server-Token"].ToString();
         
         if (string.IsNullOrEmpty(serverUrl) || string.IsNullOrEmpty(serverToken))
@@ -55,7 +58,27 @@ public static class AuthHelper
             return false;
         }
 
-        return peerCache.VerifySessionToken(serverUrl, serverToken);
+        // 1. Verify if it's a handshaked downstream peer calling us (upstream verification)
+        if (peerCache.VerifySessionToken(serverUrl, serverToken))
+        {
+            return true;
+        }
+
+        // 2. Verify if it's our configured upstream calling us (downstream verification)
+        var configuration = context.RequestServices.GetRequiredService<IConfiguration>();
+        var destUrl = (configuration["Sync:DestinationServerUrl"] ?? "").Trim().TrimEnd('/');
+
+        if (!string.IsNullOrEmpty(destUrl) && string.Equals(serverUrl, destUrl, StringComparison.OrdinalIgnoreCase))
+        {
+            // The request is coming from our upstream. Verify the token matches the one we received during handshake!
+            if (!string.IsNullOrEmpty(SyncEndpoints.UpstreamSessionToken) && 
+                string.Equals(SyncEndpoints.UpstreamSessionToken, serverToken, StringComparison.Ordinal))
+            {
+                return true;
+            }
+        }
+
+        return false;
     }
 
     // Helper to generate a server token for outgoing sync requests