perf: replace sync zip crate with async_zip in ZipService

ZipWriter<std::fs::File> performed every write_all() as a blocking
write(2) syscall on the Tokio worker thread, sequestering it for
10-100ms per file (0.8-12s for a 100-file ZIP).

Replace with async_zip::ZipFileWriter backed by a buffered
tokio::fs::File:
- All I/O (headers, deflate chunks, central directory) is fully async
- 256 KB BufWriter minimises syscall count
- Zero Tokio worker blocking during ZIP creation
- Streaming per-chunk writes keep RAM O(1) regardless of archive size
- Removed dead From<zip::result::ZipError> for DomainError impl
- zip crate retained for batch_operations.rs (separate concern)

Cargo.toml

@@ -8,7 +8,7 @@ default-run = "oxicloud"
 [dependencies]
 axum = { version = "0.8.8", features = ["multipart", "http1", "tokio", "macros"] }
 tokio = { version = "1.49.0", features = ["full"] }
-tokio-util = { version = "0.7.18", features = ["io", "codec"] }
+tokio-util = { version = "0.7.18", features = ["io", "codec", "compat"] }
 tokio-stream = { version = "0.1.18", features = ["fs"] }
 bytes = "1.11.1"
 tempfile = "3.25.0"
@@ -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_zip = { version = "0.0.18", features = ["tokio", "deflate"] }
 
 [features]
 default = []