Run node code on the venv Fluksio was installed into

The workflow this serves: make a venv, install what you work with, then `pip
install fluksio` into the same one. Building a second environment beside it
was exactly wrong — the packages the nodes need are already here, and the
Modules screen was asking for them a second time.

`NODE_VENV=auto` (the default) adopts that venv. It declines in the three
cases where adopting would be wrong: `managed` says otherwise, a managed venv
already exists and may hold packages somebody installed on purpose, or the
engine is not running from a venv at all. The images set `managed`, since the
venv in them holds the app and nothing of anybody else's.

An adopted venv is never written to. `uv pip sync` makes a venv hold exactly
the manifest, so pointed at somebody's own environment it uninstalls their
work and the engine with it — `sync()` refuses outright and `reconcile()`
returns before it can be called at startup, which is where that would have
happened first. The Modules screen lists what is installed and drops its
editor; `pip` is how that environment changes.

`fluksio serve` now names the interpreter node code runs on, which is the
thing a data scientist most needs to know at that moment. `fluksio-worker`
already defaulted `--python` to its own interpreter, so a GPU box works the
same way — that was only ever undocumented.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ue1tkFWB1bcGy3aWhCKpU

backend/Dockerfile

@@ -20,6 +20,12 @@ WORKDIR /app/
 # Ref: https://docs.astral.sh/uv/guides/integration/docker/#using-the-environment
 ENV PATH="/app/.venv/bin:$PATH"
 
+# /app/.venv holds the app and nothing of anybody else's, so there is nothing
+# here to adopt: node code gets a venv of its own on the data volume, which is
+# what the Modules screen installs into. A `pip install fluksio` into an
+# environment somebody already works in is the case that adopts instead.
+ENV NODE_VENV=managed
+
 # Install dependencies
 # Ref: https://docs.astral.sh/uv/guides/integration/docker/#intermediate-layers
 RUN --mount=type=cache,target=/root/.cache/uv \