feat(sftp): add macOS and Windows platform support (#3372)

The session watchdog now selects its detection method per
platform. It previously probed kernel TCP state through a
Linux-only procfs path, so on macOS every healthy idle session
was killed within a minute, and on Windows the watchdog never
spawned at all, leaving wedged sessions with no cleanup. Linux
keeps its fast-kill watchdog unchanged. Other platforms get a
silence-only backstop that kills a session only at the
documented 30-minute idle ceiling.

The host-key loader now has a Windows arm. It loads OpenSSH
format host keys from the configured directory and logs a
one-time warning to restrict NTFS ACLs on the key directory,
the same operator-managed approach FTPS, WebDAV, KMS, and IAM
already use on Windows. Startup previously aborted with
UnsupportedPlatform because the Unix mode-bit permission check
has no Windows equivalent. tokio's io-uring feature is now
enabled only in Linux builds. io-uring is a Linux kernel
interface and enabling it unconditionally broke the Windows
build.

Co-authored-by: houseme <housemecn@gmail.com>

crates/kms/Cargo.toml

@@ -30,7 +30,7 @@ workspace = true
 [dependencies]
 # Core dependencies
 async-trait = { workspace = true }
-tokio = { workspace = true, features = ["full","io-uring"] }
+tokio = { workspace = true, features = ["full"] }
 uuid = { workspace = true, features = ["serde"] }
 jiff = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
@@ -69,3 +69,7 @@ temp-env = { workspace = true }
 
 [features]
 default = []
+
+# io-uring is Linux-only. Scope the feature to Linux targets.
+[target.'cfg(target_os = "linux")'.dependencies]
+tokio = { workspace = true, features = ["io-uring"] }