CI pipeline with per-PR preview environments for a small product team

Testing branches is a mess for us right now: one shared staging server, a Slack channel where people call dibs, and features that sit unreviewed because staging is occupied. We are a six-person team building a B2B quoting tool (Node.js backend, React frontend, both already Dockerized). What we want built: a GitHub Actions pipeline that runs lint and tests, builds images, and spins up an isolated preview environment for every open pull request - own URL, own seeded database, torn down automatically on merge or close. AWS is our platform; we are open to ECS, Fargate or something lighter as long as you justify the cost profile in writing. A cap of roughly $150/month for preview infrastructure is part of the acceptance criteria. Documentation is half the deliverable: a README that lets any developer here debug a broken pipeline without you. Fixed-price milestones, weekly demo of progress.