feat: gsmol-compatible backup export and import behind the store feature

cli/src/main.rs

@@ -86,6 +86,14 @@ enum Command {
     Import { uri: String },
     /// List known keys and how each was learned
     Contacts,
+    /// Write a gsmol backup: sealed mail, contacts and pins, never the master
+    Export {
+        /// Output file
+        #[arg(default_value = "smol-backup.json")]
+        file: PathBuf,
+    },
+    /// Restore a gsmol backup file; merges, never overwrites a trust binding
+    ImportBackup { file: PathBuf },
     /// Issue an accept token, admit to the main tier, sync the set
     Accept { address: String },
     /// Withdraw a contact's accept token, sync the set
@@ -192,6 +200,8 @@ fn run(cli: Cli) -> Result<()> {
         Command::Resolve { address } => resolve(&store, address, cli.timeout),
         Command::Import { uri } => import(&store, uri),
         Command::Contacts => contacts(&store),
+        Command::Export { file } => export_backup(&cli.key, &store, file),
+        Command::ImportBackup { file } => import_backup(&cli.key, &store, file),
         Command::Accept { address } => accept(&cli.key, &store, address, cli.timeout),
         Command::Block { address } => block(&cli.key, &store, address, cli.timeout),
         Command::Send { .. } => send(&cli, &store),
@@ -448,6 +458,23 @@ fn contacts(store: &Store) -> Result<()> {
     Ok(())
 }
 
+fn export_backup(key: &PathBuf, store: &Store, file: &PathBuf) -> Result<()> {
+    let master = load_master(key)?;
+    let text = fumi::export::export(store, &master)?;
+    std::fs::write(file, text)?;
+    println!("wrote {}  (only the master can open it)", file.display());
+    Ok(())
+}
+
+fn import_backup(key: &PathBuf, store: &Store, file: &PathBuf) -> Result<()> {
+    let master = load_master(key)?;
+    let text = std::fs::read_to_string(file)
+        .with_context(|| format!("reading {}", file.display()))?;
+    let summary = fumi::export::import(store, &master, &text)?;
+    println!("{summary}");
+    Ok(())
+}
+
 /// An address plus the key we hold for it, for commands naming a contact.
 fn target_contact(store: &Store, text: &str) -> Result<(String, [u8; KEY_LEN])> {
     let addr = Address::parse(text)?;