HolePunch + General Improvements.

MMS/MMS.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="JetBrains.Annotations" Version="2025.2.4" />
+  </ItemGroup>
+</Project>

MMS/Models/Lobby.cs

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+using System.Collections.Concurrent;
+using System.Net.WebSockets;
+
+namespace MMS.Models;
+
+/// <summary>
+/// Client waiting for NAT hole-punch.
+/// </summary>
+public record PendingClient(string ClientIp, int ClientPort, DateTime RequestedAt);
+
+/// <summary>
+/// Game lobby. ConnectionData serves as both identifier and connection info.
+/// Steam: ConnectionData = Steam lobby ID. Matchmaking: ConnectionData = IP:Port.
+/// </summary>
+public class Lobby(
+    string connectionData,
+    string hostToken,
+    string lobbyCode,
+    string lobbyName,
+    string lobbyType = "matchmaking",
+    string? hostLanIp = null,
+    bool isPublic = true
+) {
+    /// <summary>Connection data: Steam lobby ID for Steam, IP:Port for matchmaking.</summary>
+    public string ConnectionData { get; } = connectionData;
+
+    /// <summary>Secret token for host authentication.</summary>
+    public string HostToken { get; } = hostToken;
+
+    /// <summary>Human-readable 6-character invite code.</summary>
+    public string LobbyCode { get; } = lobbyCode;
+
+    /// <summary>Display name of the lobby.</summary>
+    public string LobbyName { get; } = lobbyName;
+
+    /// <summary>Lobby type: "steam" or "matchmaking".</summary>
+    public string LobbyType { get; } = lobbyType;
+
+    /// <summary>Optional LAN IP for local network discovery.</summary>
+    public string? HostLanIp { get; } = hostLanIp;
+
+    /// <summary>Whether the lobby should appear in public browser listings.</summary>
+    public bool IsPublic { get; } = isPublic;
+
+    /// <summary>Timestamp of the last heartbeat from the host.</summary>
+    public DateTime LastHeartbeat { get; set; } = DateTime.UtcNow;
+
+    /// <summary>Queue of clients waiting for NAT hole-punch.</summary>
+    public ConcurrentQueue<PendingClient> PendingClients { get; } = new();
+
+    /// <summary>True if no heartbeat received in the last 60 seconds.</summary>
+    public bool IsDead => DateTime.UtcNow - LastHeartbeat > TimeSpan.FromSeconds(60);
+
+    /// <summary>
+    /// WebSocket connection from the host for push notifications.
+    /// </summary>
+    public WebSocket? HostWebSocket { get; set; }
+}