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/iso-store/src/lib.rs

@@ -20,6 +20,7 @@ pub mod entry;
 pub mod introspect;
 pub mod nfs_share;
 pub mod pxe_logo;
+pub mod sftp_share;
 pub mod smb;
 pub mod smb_share;
 pub mod store;
@@ -40,6 +41,13 @@ pub use smb_share::{SmbAddRequest, SmbShare, SmbShareError, SmbShareManager, Smb
 // "works in any container" property as SMB, plus support for HTTP
 // Range requests because NFSv3 READ3 takes an explicit offset.
 pub use nfs_share::{NfsAddRequest, NfsShare, NfsShareError, NfsShareManager, NfsStream};
+// v0.5.5: SFTP-over-SSH remote shares via the pure-Rust `russh` +
+// `russh-sftp` crates (ring backend — no OpenSSL, no new C deps). Like
+// NFS, supports HTTP Range requests because SFTP opens a seekable file
+// handle. See crates/iso-store/src/sftp_share.rs.
+pub use sftp_share::{
+    SftpAddRequest, SftpAuthKind, SftpShare, SftpShareError, SftpShareManager, SftpStream,
+};
 pub use store::{
     generate_boot_entries_for, slugify_str, IsoCategory, IsoMeta, IsoSource, IsoStore, UploadHandle,
 };