perf: add socket2 TCP_NODELAY + socket tuning for low-latency responses

- Replace basic TcpListener::bind with socket2 tuned socket
- TCP_NODELAY: disable Nagle's algorithm (-5 to 40ms latency on small responses)
- SO_REUSEADDR: port available immediately after server restart
- SO_REUSEPORT: ready for multi-worker scaling (Linux)
- TCP_KEEPALIVE: detect dead connections within 60s/10s interval
- listen(2048): high backlog for WebDAV connection bursts
- Eliminate redundant create_dir_all calls from upload hot path

Cargo.toml

@@ -55,6 +55,7 @@ infer = "0.19"
 async-compression = { version = "0.4", features = ["tokio", "gzip"] }
 async_zip = { version = "0.0.18", features = ["tokio", "deflate"] }
 dashmap = "6"
+socket2 = { version = "0.6.2", features = ["all"] }
 
 [features]
 default = []