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/src/state.rs

@@ -5,7 +5,9 @@ use openpxe_core::{
     AdminStore, BootLog, BrandingStore, ClientRegistry, DeploymentQueue, HostBindings, LogBus,
     Metrics, NotifyStore, SettingsStore, SsoStore,
 };
-use openpxe_iso_store::{IsoStore, NfsShareManager, SmbManager, SmbShareManager, UnattendedStore};
+use openpxe_iso_store::{
+    IsoStore, NfsShareManager, SftpShareManager, SmbManager, SmbShareManager, UnattendedStore,
+};
 use std::sync::Arc;
 use time::OffsetDateTime;
 
@@ -67,6 +69,13 @@ pub struct AppState {
     /// In-process (no subprocess); supports HTTP Range requests on
     /// NFS-sourced ISOs because NFSv3 READ3 takes an explicit offset.
     pub nfs_shares: NfsShareManager,
+    /// v0.5.5: SFTP-over-SSH share manager — pure-Rust userspace
+    /// consumer via `russh` + `russh-sftp` (ring backend, no OpenSSL).
+    /// Ships alongside SMB/NFS as the third remote-library protocol.
+    /// In-process (no subprocess, no kernel mount); supports HTTP Range
+    /// requests because SFTP opens a seekable file handle. Authenticates
+    /// the server's SSH host key on a trust-on-first-use basis.
+    pub sftp_shares: SftpShareManager,
     /// v0.5.2: uploaded unattended-install answer files (Kickstart /
     /// Preseed / Autoinstall / Windows answer files). Served on demand to
     /// booting clients with per-host hostname/IP/MAC templating; lives in