Add scheduled jobs (crons)

Workflow

1. Create convex/crons.ts with cronJobs().
2. Schedule internal functions (never public api.*) at the right interval.
3. Make handlers idempotent (safe to re-run); keep each run small.
4. Verify the job appears in the dashboard schedule.

Rules

• Schedule internal.* functions, never api.*.
• Keep cron handlers small + idempotent.
• Don't poll tight intervals for things a subscription can push.