feat: add db state persistence

server/src/persistence.rs

@@ -0,0 +1,15 @@
+use std::path::PathBuf;
+use tokio::{fs, io};
+use minisql::interpreter::State;
+
+pub async fn state_from_file(path: &PathBuf) -> io::Result<State> {
+    let content = fs::read_to_string(path).await?;
+    let state = serde_json::from_str(&content)?;
+    Ok(state)
+}
+
+pub async fn state_to_file(state: &State, path: &PathBuf) -> io::Result<()> {
+    let content = serde_json::to_string(state)?;
+    fs::write(path, content).await?;
+    Ok(())
+}