feature: preview renders + massive refactor for enabling it

[Walther]

• adds a new worker type preview
• preview tasks are pushed to and pulled from a redis queue
• preview results are pushed to a redis set
• regular render tasks work as before with a redis queue and postgres storage (refactor later to postgres only?)
• frontend has primitive preview showing capability with an auto-refreshing react component
• lots and lots of refactoring to enable this in the first place
• a bunch of bad choices and dead ends before finding the happy path
• 7 hours of work for what might look like a simple feature!