Function init 

pub async fn init(database_url: &str) -> Result<DurableInstance, DurableError>

Initialize durable: connect to Postgres, run migrations, start heartbeat, and recover stale tasks from prior crashes.

Returns a DurableInstance which derefs to DatabaseConnection for backwards compatibility.

Uses HeartbeatConfig::default() (60 s heartbeat, 180 s staleness). For custom intervals use init_with_config.

let durable = durable::init("postgres://localhost/mydb").await?;