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/fluksio/core/config.py

@@ -70,6 +70,12 @@ class Settings(BaseSettings):
     # alerting is: where a screen hangs is the deployment's concern rather than
     # any one dashboard's.
     PANELS_FILE: Path = Path("flow-data/panels.json")
+    # Which interpreter node code runs on. "auto" adopts the venv the engine
+    # was installed into, when it was installed into one and there is no venv
+    # of its own to lose — which is the `pip install fluksio` beside your own
+    # packages case. "managed" always builds a separate one, which is what a
+    # container wants. A path names an interpreter outright.
+    NODE_VENV: str = "auto"
     # The MCP endpoint, and the OAuth server agents authenticate against. Off
     # until someone asks for it: it opens client registration to the network.
     MCP_ENABLED: bool = False