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/store.rs

@@ -24,6 +24,10 @@ use tokio::io::AsyncWriteExt;
 /// `nfs3_client` crate (in-process, no subprocess). Same "works in
 /// any container" property as SMB, plus Range requests work because
 /// NFSv3 READ3 takes an explicit offset.
+/// `Sftp` (v0.5.5) — remote SFTP-over-SSH share, streamed via the
+/// pure-Rust `russh` + `russh-sftp` crates (in-process). Like NFS it
+/// supports HTTP Range requests because SFTP opens a seekable file
+/// handle (`SSH_FXP_READ` at offset).
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(tag = "kind", rename_all = "snake_case")]
 pub enum IsoSource {
@@ -42,6 +46,13 @@ pub enum IsoSource {
         /// Filename at the export root.
         relative_path: String,
     },
+    /// v0.5.5: SFTP-over-SSH via the in-process `russh` + `russh-sftp`
+    /// crates.
+    Sftp {
+        share_id: String,
+        /// Filename at the export root.
+        relative_path: String,
+    },
 }
 
 /// Where the ISO lands in the PXE menu hierarchy.
@@ -299,11 +310,11 @@ impl IsoStore {
                     None
                 }
             }
-            // SMB and NFS sources have no local path — they're
+            // SMB, NFS, and SFTP sources have no local path — they're
             // streamed in-process. Callers must inspect the source
             // kind first and dispatch to the appropriate share
             // manager.
-            IsoSource::Smb { .. } | IsoSource::Nfs { .. } => None,
+            IsoSource::Smb { .. } | IsoSource::Nfs { .. } | IsoSource::Sftp { .. } => None,
         }
     }
 
@@ -363,9 +374,9 @@ impl IsoStore {
     pub fn drop_external_source(&self, share_id: &str) {
         let mut g = self.inner.write();
         g.isos.retain(|_, m| match &m.source {
-            IsoSource::Smb { share_id: sid, .. } | IsoSource::Nfs { share_id: sid, .. } => {
-                sid != share_id
-            }
+            IsoSource::Smb { share_id: sid, .. }
+            | IsoSource::Nfs { share_id: sid, .. }
+            | IsoSource::Sftp { share_id: sid, .. } => sid != share_id,
             IsoSource::Local => true,
         });
     }
@@ -659,7 +670,10 @@ mod tests {
         // good.
         let s = linux_cmdline(DistroFamily::DebianUbuntu, "ubuntu-24-04");
         assert!(s.contains("boot=casper"), "{s}");
-        assert!(s.contains("iso-url=${base-url}/iso/ubuntu-24-04.iso"), "{s}");
+        assert!(
+            s.contains("iso-url=${base-url}/iso/ubuntu-24-04.iso"),
+            "{s}"
+        );
         assert!(s.contains("ds=nocloud"), "{s}");
         assert!(s.contains("ip=dhcp"), "{s}");
         assert!(!s.contains("netboot=url"), "legacy option leaked: {s}");