Recurring Tasks: schedule daily/weekly task generation

[TheSethRose]

Summary

Add first-class recurring tasks with automatic generation and visibility in the UI.

Current behavior

• Tasks are one-off records in the tasks table; there is no recurrence metadata in the schema documented in docs/DATABASE.md.
• Task creation/update flows are handled through src/app/api/tasks/route.ts and UI in src/app/(dashboard)/tasks/TaskModal.tsx.
• The existing background worker (scripts/recurring-work.js) syncs GitHub issues and performs git safety pushes, but it does not create recurring tasks.

Proposed behavior

• Introduce a recurrence model (either a new task_recurrences table or columns on tasks) with fields such as rrule, next_run_at, last_run_at, timezone, enabled, and source_task_id.
• Add server-side generation that creates the next instance at schedule time and updates next_run_at (cron or worker-friendly).
• Update the task creation/edit UI to set recurrence rules and show the upcoming run in the task detail view.
• Ensure the kanban view shows generated instances in the correct status and project.

Scope

• Database migration for recurrence storage.
• API: extend POST/PUT to accept recurrence settings; add a lightweight endpoint to compute next run.
• Worker: scheduled job to create next instance (use existing worker patterns).
• UI: recurrence picker in TaskModal and display of upcoming run.

Acceptance criteria

• Recurring tasks can be created/edited with hourly, daily, weekly, monthly, and yearly cadences, as well as custom intervals and timezone support.
• A scheduled worker creates a new task instance at or after the next run time.
• Generated instances inherit project, tags, priority, and notes template.
• Recurring rules can be paused without deleting historical instances.
• No duplicate generation when the worker runs multiple times (idempotent by (recurrence_id, run_at)).

Open questions

• What should be the default creation window (exact time vs. start-of-day)?
• Should recurrence live on the base task or a separate recurrence entity?

References

• docs/DATABASE.md (task schema)
• src/app/api/tasks/route.ts (task CRUD)
• src/app/(dashboard)/tasks/TaskModal.tsx (task form)
• scripts/recurring-work.js (background worker pattern)