Support custom, persistent environments

Allow teams to create named environments beyond Development, Preview, Staging, and Production, each with independent deployments, API keys, environment variables, and runs.

Teams may need long-lived environments for workflows such as QA, acceptance testing, or internal validation, with control over when each receives updates.

Preview branches offer a workaround, but their branch-based lifecycle and additional request headers add complexity. Native custom environments would provide a simpler way to manage persistent, isolated workloads.