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/test_metrics.py

@@ -126,6 +126,56 @@ def test_events_become_rollups_failures_runs_and_audit(db: Session) -> None:
     assert open_run.status == "running"
 
 
+def test_a_flush_the_database_refused_is_written_by_the_next_one(
+    db: Session, monkeypatch
+) -> None:
+    collector = MetricsCollector(EventBus())
+    ts = datetime.now(timezone.utc).replace(second=0, microsecond=0).timestamp()
+    collector.handle(
+        {
+            "type": "audit",
+            "action": "held back",
+            "flow": FLOW,
+            "user": "held@example.com",
+            "ts": ts,
+        }
+    )
+
+    def refuse(*_args: object) -> None:
+        raise RuntimeError("the database is gone")
+
+    monkeypatch.setattr(collector, "_write", refuse)
+    asyncio.run(collector.flush())
+    monkeypatch.undo()
+    asyncio.run(collector.flush())
+
+    audit = db.exec(
+        select(EngineEvent).where(EngineEvent.actor == "held@example.com")
+    ).one()
+    assert audit.detail == "held back"
+
+
+def test_a_node_id_wider_than_the_column_still_records(db: Session) -> None:
+    collector = MetricsCollector(EventBus())
+    ts = datetime.now(timezone.utc).replace(second=0, microsecond=0).timestamp()
+    long_node = f"{FLOW}.{'w' * 400}"
+    collector.handle(
+        {
+            "type": "node_executed",
+            "flow": FLOW,
+            "node": long_node,
+            "duration_ms": 1.0,
+            "ts": ts,
+        }
+    )
+    asyncio.run(collector.flush())
+
+    bucket = db.exec(
+        select(MetricBucket).where(MetricBucket.node == long_node[:255])
+    ).one()
+    assert bucket.executions == 1
+
+
 def test_a_run_that_did_not_fail_reads_ok(db: Session) -> None:
     collector = MetricsCollector(EventBus())
     ts = time.time()