Recurring tasks (MVP)

Add nullable `cron_expr TEXT`. If present, after execution, compute next `run_at`. Validate with a cron parser.

Add a nullable `cron_expr TEXT` column to the tasks table. After a task executes, compute the next `run_at` using a cron parser. Look at the existing task execution logic and database models to understand where to add the cron validation and scheduling logic. Check for existing cron parsing libraries in the project dependencies.

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