feat(jetstream): add mirror sync loop and source coordination with filtering (C9+C10)

src/NATS.Server/JetStream/Models/StreamConfig.cs

@@ -35,4 +35,12 @@ public sealed class StreamSourceConfig
     public string Name { get; set; } = string.Empty;
     public string? SubjectTransformPrefix { get; set; }
     public string? SourceAccount { get; set; }
+
+    // Go: StreamSource.FilterSubject — only forward messages matching this subject filter.
+    public string? FilterSubject { get; set; }
+
+    // Deduplication window in milliseconds for Nats-Msg-Id header-based dedup.
+    // Defaults to 0 (disabled). When > 0, duplicate messages with the same Nats-Msg-Id
+    // within this window are silently dropped.
+    public int DuplicateWindowMs { get; set; }
 }