v0.5.5: SFTP-over-SSH remote shares (russh, pure-Rust, ring backend)

Adds SFTP as a third remote ISO-library protocol alongside SMB and NFS.
Pure-Rust russh + russh-sftp on the ring crypto backend — no kernel
mount, no subprocess, no OpenSSL, no new C deps. Like NFS (and unlike
SMB), SFTP-sourced ISOs support HTTP Range requests because SFTP opens
a seekable file handle.

- iso-store: SftpShareManager (connect/auth/READDIR/seekable stream),
  IsoSource::Sftp, password OR SSH-key auth, trust-on-first-use host-key
  pinning, 0600 credential sidecar with a restart-safe derived path.
- http-api: /api/sftp-shares routes, Range-aware ISO dispatch arm,
  status/metrics counts, /api/docs entry, `sftp` terminal commands.
- webui: "SFTP (SSH)" protocol option with a password/key auth toggle,
  host-key fingerprint display, dashboard tile, updated copy.

SCP was deliberately rejected: sequential-only (no Range) and its crates
wrap libssh2 (C + OpenSSL), which would break the static-musl build.

russh is pinned to =0.55.0: russh 0.61 needs the stable RustCrypto
generation (pkcs8 0.11), which is API-incompatible with the release-
candidate crates bergshamra-crypto pins (pkcs8 =0.11.0-rc.11). 0.55 is
the newest russh on the prior generation (pkcs8 0.7) that coexists. Do
not bump past 0.55 until bergshamra adopts stable RustCrypto.

252 tests pass, clippy clean, static musl x86_64 binary (ring already
present via rustls + bergshamra, so no new crypto/C deps).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/http-api/tests/full_flow.rs

@@ -14,7 +14,7 @@ use axum::body::Body;
 use axum::http::{header, Request, StatusCode};
 use openpxe_core::{ClientRegistry, DeploymentQueue, HostBindings, LogBus, Metrics, SettingsStore};
 use openpxe_http_api::{build_router, AppState};
-use openpxe_iso_store::{IsoStore, NfsShareManager, SmbShareManager};
+use openpxe_iso_store::{IsoStore, NfsShareManager, SftpShareManager, SmbShareManager};
 use tempfile::tempdir;
 use tower::ServiceExt;
 
@@ -96,6 +96,7 @@ async fn build_state() -> (AppState, tempfile::TempDir) {
     let settings = SettingsStore::load_or_default(dir.path());
     let smb_shares = SmbShareManager::new(dir.path(), iso_store.clone());
     let nfs_shares = NfsShareManager::new(dir.path(), iso_store.clone());
+    let sftp_shares = SftpShareManager::new(dir.path(), iso_store.clone());
     let unattended = openpxe_iso_store::UnattendedStore::new(dir.path().join("unattended"));
     unattended.ensure_dir().await.unwrap();
     let log_bus = LogBus::new(64);
@@ -124,6 +125,7 @@ async fn build_state() -> (AppState, tempfile::TempDir) {
         smb: None,
         smb_shares,
         nfs_shares,
+        sftp_shares,
         unattended,
         uploads: openpxe_http_api::uploads::UploadSessions::default(),
         log_bus,