feat: enforce jetstream retention and limits

src/NATS.Server/JetStream/Storage/FileStore.cs

@@ -62,6 +62,17 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         });
     }
 
+    public void TrimToMaxMessages(ulong maxMessages)
+    {
+        while ((ulong)_messages.Count > maxMessages)
+        {
+            var first = _messages.Keys.Min();
+            _messages.Remove(first);
+        }
+
+        RewriteDataFile();
+    }
+
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 
     private void LoadExisting()
@@ -91,6 +102,22 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         }
     }
 
+    private void RewriteDataFile()
+    {
+        var lines = new List<string>(_messages.Count);
+        foreach (var message in _messages.OrderBy(kv => kv.Key).Select(kv => kv.Value))
+        {
+            lines.Add(JsonSerializer.Serialize(new FileRecord
+            {
+                Sequence = message.Sequence,
+                Subject = message.Subject,
+                PayloadBase64 = Convert.ToBase64String(message.Payload.ToArray()),
+            }));
+        }
+
+        File.WriteAllLines(_dataFilePath, lines);
+    }
+
     private sealed class FileRecord
     {
         public ulong Sequence { get; init; }