feat: mount spaces/folders over SFTP with SSH-key auth

Add an in-process SFTP server (russh) backed directly by the nodes
table, so a space can be edited from nvim or synced with sshfs without
a separate filesystem mirror. Access is authenticated by public key
against a new ssh_keys table, managed via :sshkey in the app; the
feature stays off until WORKSPACE_SFTP_BIND is set.

src/auth.rs

@@ -100,7 +100,7 @@ impl FromRequestParts<AppState> for Session {
 #[cfg(test)]
 mod tests {
     use super::{credentials_match, issue_token, verify_token};
-    use crate::config::{BackupConfig, Config, MailConfig};
+    use crate::config::{BackupConfig, Config, MailConfig, SftpConfig};
 
     fn config(session_days: i64) -> Config {
         Config {
@@ -128,6 +128,7 @@ mod tests {
                 digest_at: None,
                 kinds: Vec::new(),
             },
+            sftp: SftpConfig { bind: None },
         }
     }