feat(files): streaming ZIP download in the project explorer

Each row of the project Files tab gains a download action, and the toolbar
gains a Download ZIP button scoped to the folder being browsed (at the root,
the whole project). Visible to read-only members too: download is a read.

Single files need no new backend: they reuse GET /api/file?force_download.
Directories go through the new GET /api/file/download, which builds the ZIP
on the fly: an async task walks the tree and async_zip (Astral's maintained
rs-async-zip fork) streams entries into a bounded duplex stream backing the
response body — no temp file, no whole-archive buffer, backpressure for free,
and the task dies with the client. Compression is per entry: Deflate at
maximum level, except files whose magic bytes name an already-compressed
format (media/PDF via the shared sniffer, the ZIP family, gzip/zstd/7z/rar,
compressed audio), which are Stored. Entries are prefixed with the folder
name, empty folders and unix permission bits survive, symlinks are never
followed into the archive, and containment stays fail-closed under the
resolved root. Covered by a round-trip test read back with the crate's own
reader (and verified against unzip/python's zipfile).

Cargo.lock

@@ -146,6 +146,21 @@ version = "0.7.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7c02d123df017efcdfbd739ef81735b36c5ba83ec3c59c80a9d7ecc718f92e50"
 
+[[package]]
+name = "astral_async_zip"
+version = "0.0.20"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bd939d79959c3f49a648a1d7857d63cc62548725a6b060b8dbf0ea5c92470b63"
+dependencies = [
+ "async-compression",
+ "crc32fast",
+ "futures-lite",
+ "pin-project",
+ "thiserror",
+ "tokio",
+ "tokio-util",
+]
+
 [[package]]
 name = "async-compression"
 version = "0.4.41"
@@ -154,6 +169,7 @@ checksum = "d0f9ee0f6e02ffd7ad5816e9464499fba7b3effd01123b515c41d1697c43dad1"
 dependencies = [
  "compression-codecs",
  "compression-core",
+ "futures-io",
  "pin-project-lite",
  "tokio",
 ]
@@ -1327,6 +1343,19 @@ version = "0.3.32"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cecba35d7ad927e23624b22ad55235f2239cfa44fd10428eecbeba6d6a717718"
 
+[[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 = "0.3.32"
@@ -4181,6 +4210,7 @@ name = "skald"
 version = "0.2.0"
 dependencies = [
  "anyhow",
+ "astral_async_zip",
  "async-trait",
  "axum",
  "chrono",
@@ -5007,6 +5037,7 @@ checksum = "9ae9cec805b01e8fc3fd2fe289f89149a9b66dd16786abd8b19cfa7b48cb0098"
 dependencies = [
  "bytes",
  "futures-core",
+ "futures-io",
  "futures-sink",
  "futures-util",
  "pin-project-lite",