feat: add per-account connection/subscription limits with AccountConfig

src/NATS.Server/Auth/Account.cs

@@ -10,8 +10,11 @@ public sealed class Account : IDisposable
     public string Name { get; }
     public SubList SubList { get; } = new();
     public Permissions? DefaultPermissions { get; set; }
+    public int MaxConnections { get; set; } // 0 = unlimited
+    public int MaxSubscriptions { get; set; } // 0 = unlimited
 
     private readonly ConcurrentDictionary<ulong, byte> _clients = new();
+    private int _subscriptionCount;
 
     public Account(string name)
     {
@@ -19,10 +22,31 @@ public sealed class Account : IDisposable
     }
 
     public int ClientCount => _clients.Count;
+    public int SubscriptionCount => Volatile.Read(ref _subscriptionCount);
 
-    public void AddClient(ulong clientId) => _clients[clientId] = 0;
+    /// <summary>Returns false if max connections exceeded.</summary>
+    public bool AddClient(ulong clientId)
+    {
+        if (MaxConnections > 0 && _clients.Count >= MaxConnections)
+            return false;
+        _clients[clientId] = 0;
+        return true;
+    }
 
     public void RemoveClient(ulong clientId) => _clients.TryRemove(clientId, out _);
 
+    public bool IncrementSubscriptions()
+    {
+        if (MaxSubscriptions > 0 && Volatile.Read(ref _subscriptionCount) >= MaxSubscriptions)
+            return false;
+        Interlocked.Increment(ref _subscriptionCount);
+        return true;
+    }
+
+    public void DecrementSubscriptions()
+    {
+        Interlocked.Decrement(ref _subscriptionCount);
+    }
+
     public void Dispose() => SubList.Dispose();
 }

src/NATS.Server/Auth/AccountConfig.cs

@@ -0,0 +1,8 @@
+namespace NATS.Server.Auth;
+
+public sealed class AccountConfig
+{
+    public int MaxConnections { get; init; }      // 0 = unlimited
+    public int MaxSubscriptions { get; init; }    // 0 = unlimited
+    public Permissions? DefaultPermissions { get; init; }
+}