perf: replace buffered decompress_stream with async-compression streaming

- Add async-compression dependency with tokio+gzip features
- Rewrite decompress_stream to use true streaming pipeline:
  Stream<Bytes> → StreamReader → BufReader(64KB) → GzipDecoder → ReaderStream(64KB)
- Memory usage drops from ~4GB (1GB compressed file) to constant ~128KB
- Eliminates OOM risk on large compressed file decompression

Cargo.lock

@@ -1820,6 +1820,7 @@ version = "0.4.2"
 dependencies = [
  "anyhow",
  "argon2",
+ "async-compression",
  "async-stream",
  "async-trait",
  "async_zip",