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.toml

@@ -49,6 +49,7 @@ reqwest = { version = "0.12", default-features = false, features = ["json", "rus
 base64 = "0.22.1"
 fs2 = "0.4"
 rayon = "1.10"
+async-compression = { version = "0.4", features = ["tokio", "gzip"] }
 async_zip = { version = "0.0.18", features = ["tokio", "deflate"] }
 
 [features]