Implemented with Antigravity.

Server/Security/PeerCache.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+
+namespace SNote.Server.Security;
+
+public class PeerCache
+{
+    private readonly ConcurrentDictionary<string, byte> _downstreamPeers = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ConcurrentDictionary<string, DateTime> _recentBroadcastIds = new(StringComparer.OrdinalIgnoreCase);
+
+    public void RegisterPeer(string peerUrl)
+    {
+        if (string.IsNullOrWhiteSpace(peerUrl)) return;
+        
+        var cleanUrl = peerUrl.Trim().TrimEnd('/');
+        _downstreamPeers.TryAdd(cleanUrl, 0);
+        Console.WriteLine($"[PeerCache] Registered downstream peer node: {cleanUrl}");
+    }
+
+    public void RemovePeer(string peerUrl)
+    {
+        if (string.IsNullOrWhiteSpace(peerUrl)) return;
+        
+        var cleanUrl = peerUrl.Trim().TrimEnd('/');
+        if (_downstreamPeers.TryRemove(cleanUrl, out _))
+        {
+            Console.WriteLine($"[PeerCache] Evicted offline peer node: {cleanUrl}");
+        }
+    }
+
+    public List<string> GetPeers()
+    {
+        return new List<string>(_downstreamPeers.Keys);
+    }
+
+    public bool TryProcessBroadcast(string broadcastId)
+    {
+        if (string.IsNullOrWhiteSpace(broadcastId)) return false;
+
+        // Clean up old entries to prevent infinite memory growth (older than 10 minutes)
+        var cutoff = DateTime.UtcNow.AddMinutes(-10);
+        foreach (var kvp in _recentBroadcastIds)
+        {
+            if (kvp.Value < cutoff)
+            {
+                _recentBroadcastIds.TryRemove(kvp.Key, out _);
+            }
+        }
+
+        return _recentBroadcastIds.TryAdd(broadcastId, DateTime.UtcNow);
+    }
+}