Take the engine off the path node code imports from, and let it stop

The worker script is handed to the interpreter by path, so app/flow was
sys.path[0] for every node: `import queue` got the engine's. It now drops
its own directory before anything else imports, and runs with the
deployment's credentials scrubbed out of its environment.

Also: reload builds off the event loop, the pool wakes what is blocked on
it when it stops, a refused metrics flush is kept for the next one rather
than dropped, and the cascade events are paired through failures.

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

backend/tests/flow/test_modules.py

@@ -52,6 +52,21 @@ def test_a_manifest_already_applied_is_not_installed_again(
     assert (venv / ".applied").read_text() == marker
 
 
+def test_a_missing_uv_is_a_failed_apply_rather_than_a_crash(
+    venv: Path, monkeypatch: pytest.MonkeyPatch
+):
+    def no_uv(*args: object, **kwargs: object) -> None:
+        raise FileNotFoundError(2, "No such file or directory: 'uv'")
+
+    monkeypatch.setattr(subprocess, "run", no_uv)
+    ok, output = modules.sync("")
+
+    # The route turns this into a 400 with the output; an exception would be a
+    # 500 with nothing in it.
+    assert ok is False
+    assert "uv could not run" in output
+
+
 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")