Per-branch workers

Implement per-branch workers to enable trouble-free, it-just-works previews.

Currently, if the correct book.yml isn't in place in `main` in the repo, preview pages can fail to load. Although there's a well-known work-around, the users for previews are often not very technical and/or don't know about, or even care about how the backend works. In this case, it takes a number of non-obvious steps to get previews working.

We're open to a paid CloudFlare workers plan, if necessary.

The issue names no files, tests, or entry points. Start by tracing the current preview flow and how the branch's book.yml is resolved when it is absent from main. Done means previews work per branch without the documented workaround, with any required CloudFlare Workers plan identified.

Written by the indexing model from the issue text.