n1x-backend-api-fastapi-runtime

`backend-python` ships the FastAPI runtime to every Python project, whether or
not `backend-api` is selected: `_lib/service.py`, the `fastapi`, `uvicorn` and
`pydantic` dependencies, and a Dockerfile `CMD` that starts `api.main:app`.

Make `backend-api` own the FastAPI runtime. A project generated without
`backend-api` should get none of it (no FastAPI code, no FastAPI packages, no
API start command), and everything else in its backend should keep working as
it does today. A project with `backend-api` should keep a working API that
still deploys and runs on Cloud Run as it does today.

## Environment

This is the n1x repository itself: it has no app or database of its own to
run. `node src/cli.mjs init <dir> --plugins <a,b>` (or `apply --project
<dir>` over an `n1x.yaml`) renders a project to try. `just`, `uv` with
Python 3.13, and `terraform` are installed, and the Terraform providers a
generated project's `infra/` uses (google, github and random) come from a
local mirror, so `terraform init` works offline in a rendered `infra/`. There
are no Google Cloud or GitHub credentials here, and no `gcloud` or `docker`.