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/tests/__init__.py

@@ -31,6 +31,11 @@ os.environ.setdefault("EMAILS_FROM_EMAIL", "noreply@example.com")
 # builds a TestClient — and so a lifespan — per test module. Tests that want the
 # endpoint mount it themselves.
 os.environ["MCP_ENABLED"] = "false"
+# The suite runs from a venv of its own, which "auto" would adopt as if it were
+# a data scientist's — so node code would run on the checkout's environment and
+# the module tests would have nothing of their own to build. The tests that
+# cover adoption ask for it explicitly.
+os.environ["NODE_VENV"] = "managed"
 # The private seeding endpoints are opt-in; the suite is one of the two places
 # (with the dev stack) where they are meant to work.
 os.environ["PRIVATE_API_ENABLED"] = "true"