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.lock

@@ -99,6 +99,7 @@ checksum = "d10e4f991a553474232bc0a31799f6d24b034a84c0971d80d2e2f78b2e576e40"
 dependencies = [
  "compression-codecs",
  "compression-core",
+ "futures-io",
  "pin-project-lite",
  "tokio",
 ]
@@ -147,6 +148,21 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "async_zip"
+version = "0.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d8c50d65ce1b0e0cb65a785ff615f78860d7754290647d3b983208daa4f85e6"
+dependencies = [
+ "async-compression",
+ "crc32fast",
+ "futures-lite",
+ "pin-project",
+ "thiserror",
+ "tokio",
+ "tokio-util",
+]
+
 [[package]]
 name = "atoi"
 version = "2.0.0"
@@ -920,6 +936,19 @@ version = "0.3.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9e5c1b78ca4aae1ac06c48a526a655760685149f0d465d21f37abfe57ce075c6"
 
+[[package]]
+name = "futures-lite"
+version = "2.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f78e10609fe0e0b3f4157ffab1876319b5b0db102a2c60dc4626306dc46b44ad"
+dependencies = [
+ "fastrand",
+ "futures-core",
+ "futures-io",
+ "parking",
+ "pin-project-lite",
+]
+
 [[package]]
 name = "futures-macro"
 version = "0.3.31"
@@ -1793,6 +1822,7 @@ dependencies = [
  "argon2",
  "async-stream",
  "async-trait",
+ "async_zip",
  "axum",
  "base64",
  "bytes",
@@ -1933,6 +1963,26 @@ version = "2.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
+[[package]]
+name = "pin-project"
+version = "1.1.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "677f1add503faace112b9f1373e43e9e054bfdd22ff1a63c1bc485eaec6a6a8a"
+dependencies = [
+ "pin-project-internal",
+]
+
+[[package]]
+name = "pin-project-internal"
+version = "1.1.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e918e4ff8c4549eb882f14b3a4bc8c8bc93de829416eacf579f1207a8fbf861"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "pin-project-lite"
 version = "0.2.16"
@@ -3060,6 +3110,7 @@ checksum = "9ae9cec805b01e8fc3fd2fe289f89149a9b66dd16786abd8b19cfa7b48cb0098"
 dependencies = [
  "bytes",
  "futures-core",
+ "futures-io",
  "futures-sink",
  "pin-project-lite",
  "tokio",