feat(cli): status shows last export age

Reads .relicario/last_backup (written by cmd_backup_export). Format:
'never' for fresh vaults, '4 days ago' otherwise. Closes the
'is my backup stale?' question without leaving the terminal.

crates/relicario-cli/src/main.rs

@@ -1918,11 +1918,32 @@ fn cmd_status() -> Result<()> {
         .map(|s| s.trim().to_string())
         .unwrap_or_else(|| "(no commits)".into());
 
+    // Last backup age (read from marker written by cmd_backup_export).
+    let last_backup_path = vault.root().join(".relicario").join("last_backup");
+    let last_backup_str = if last_backup_path.exists() {
+        let line = std::fs::read_to_string(&last_backup_path)
+            .unwrap_or_default()
+            .trim()
+            .to_string();
+        // Parse the ISO-8601 we wrote in cmd_backup_export.
+        match chrono::DateTime::parse_from_rfc3339(&line) {
+            Ok(then) => {
+                let now = relicario_core::now_unix();
+                let age = now - then.timestamp();
+                crate::helpers::humanize_age(age.max(0))
+            }
+            Err(_) => "unknown".to_string(),
+        }
+    } else {
+        "never".to_string()
+    };
+
     println!("Vault:    {}", root.display());
     println!("Items:    {total_items} total ({active_items} active, {trashed_items} trashed)");
     println!("Attachments: {attachment_count} ({attachment_bytes} bytes)");
     println!("Devices:  {device_count}");
     println!("Last commit: {last_commit}");
+    println!("Last export: {last_backup_str}");
     Ok(())
 }
 fn cmd_device(action: DeviceAction) -> Result<()> {