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/pyproject.toml

@@ -89,6 +89,9 @@ ignore = [
 # Printing is what this one is about: node code is user code, and `print` is
 # how it says things.
 "tests/flow/test_logs.py" = ["ARG001", "T201"]
+# This one takes its own directory off sys.path before the rest of its imports
+# run, which is the whole point of doing it there.
+"app/flow/worker_main.py" = ["E402"]
 
 [tool.ruff.lint.pyupgrade]
 # Preserve types, even if a file imports `from __future__ import annotations`.