Support Fly Preview environment for PR's

[kasperpeulen]

Fly supports PR/branch preview environments (review apps):
https://fly.io/docs/blueprints/review-apps-guide/
https://fly.io/docs/django/advanced-guides/staging-environments-with-github-actions/

Right now Epic Stack only deploys staging from dev. That works, but a single shared staging becomes a bottleneck and a noisy signal (“did QA fail because of my PR or because dev changed from another PR?”).

With per-PR previews you get:

• test the PR, not “whatever is on dev”
• parallel QA (no staging coordination / no stomping)
• one-click review for maintainers + non-dev reviewers
• better validation of real deploy/runtime behavior than when only tested on local dev

[kentcdodds]

I've wanted to do sobering like this for a long time. Didn't realize they had formal support for it. I'd welcome a PR!

[kasperpeulen]

Implemented here:
#1070

And tested on my own fork as forks don't have access to secrets:
https://github.com/kasperpeulen/epic-stack