Add scheduler for running playbooks

[tsg]

This is stacked on top of #4. See commits starting with 4faca9b.

This PR adds a simple scheduler / task queue that saves it's data in Postgres.

At the high level, it works like this:

• "something" periodically calls a Next.js route to give us a clock tick. This can be a vercel cron that fires every 1 minute, or anything else. For local development, I added pnpm run dev-scheduler that calls the tick route every 10 seconds.
• when this route is called, we look for jobs that are ready to be run. If they are, we start up to 20 of them in parallel. If there are more than 20, we leave the rest for the next run.
• after a run, we compute when we want the next run to be (e.g. based on the cron expression) and store that in the DB.

This is a simplistic queue but I think will do the job for now. The only thing that i'm not sure is how we'll call that private tick route when deploying as OSS. Maybe actual cron in the docker-container.