Periodic executor dispatch

[Southclaws]

My main use-case for Pico is to run docker-compose up -d for config changes. However, if a container dies for some reason, bringing it back is done in two ways:

• manually ssh into the machine, cd into its target directory and manually run compose (ew!) - this is really annoying/insecure when the container depends on a bunch of secrets from Vault - you have to copy those into the command line or .env file (and risk forgetting to remove it after)
• push an empty commit to the config repo to trigger pico - the slightly safer option, but clutters your history and is a bit of a hack

One of the goals of the web UI (#10) is to provide a way to do this with a click instead of an ssh.

One of the goals of using an event bus (#27) is to allow multiple trigger sources for command execution.

It would make sense to introduce another provider (I guess we're calling event sources "providers" now?) that's based on periodic automated events (cron-tab style) alongside the Git events. So a target will react to Git pushes, but also run its command every now and then anyway.

At first, this felt like feature creep. I didn't really want to move towards a kubeapplier style reconcillation system because that would be very Docker-specific (but maybe...). I gave it some thought and realised this could be a useful solution that applies outside of Docker use-cases (periodically run a container, or a webhook, or any process, all configured via Git instead of via crontab where it gets buried in machine-specific config)