Run python nodes out of process, with modules of their own

User code no longer execs in the engine. A pool of persistent worker
subprocesses speaks one JSON object per line; the controller installs a
proxy as the node's function, so every execution path funnels through it
and the pipeline is untouched. A crash costs one subprocess, a per-node
timeout is a kill, and cancelling from the canvas is that same kill.

The workers run a venv of the user's own on the data volume, filled from
a pip manifest versioned beside the flows. Applying it retires the
workers and rebuilds, so a package lands without restarting the engine.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

backend/tests/flow/test_modules.py

@@ -0,0 +1,62 @@
+"""The venv node code imports from is built and kept in step with a manifest."""
+
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from app.flow import modules
+from app.flow.store import FlowStore
+
+pytestmark = pytest.mark.skipif(
+    shutil.which("uv") is None, reason="module management needs uv"
+)
+
+
+@pytest.fixture
+def venv(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
+    directory = tmp_path / "user-venv"
+    monkeypatch.setattr(modules, "VENV_DIR", directory)
+    return directory
+
+
+def test_an_empty_manifest_gives_a_venv_of_its_own(venv: Path, tmp_path: Path):
+    store = FlowStore(tmp_path / "flows")
+    store.write_requirements("")
+    modules.sync("")
+
+    assert Path(modules.venv_python()).exists()
+    assert modules.venv_python().startswith(str(venv))
+
+    information = modules.info(store)
+    assert information.applied is True
+    assert information.python_version
+
+
+def test_a_manifest_already_applied_is_not_installed_again(
+    venv: Path, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    store = FlowStore(tmp_path / "flows")
+    store.write_requirements("")
+    modules.sync("")
+    marker = (venv / ".applied").read_text()
+
+    # Recorded rather than refused: reconcile swallows what it fails on, so an
+    # exception raised in here would never reach the test.
+    calls: list[object] = []
+    monkeypatch.setattr(subprocess, "run", lambda *args, **kwargs: calls.append(args))
+    modules.reconcile(store)
+
+    assert calls == []
+    assert (venv / ".applied").read_text() == marker
+
+
+def test_a_manifest_that_does_not_resolve_leaves_the_venv_alone(venv: Path):
+    modules.sync("")
+    ok, output = modules.sync("fluksio-no-such-package-anywhere==9.9.9")
+
+    assert ok is False
+    assert output
+    # The marker still describes the manifest that actually installed.
+    assert (venv / ".applied").read_text() == modules._digest("")