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

frontend/src/client/schemas.gen.ts

@@ -1308,7 +1308,7 @@ export const NodeDef_InputSchema = {
                 }
             ],
             title: 'Timeout',
-            description: "Seconds this node's code may run before it is stopped. Above 60 the engine may deliver its work again while it is still running."
+            description: "Seconds this node's code may run before it is stopped. This covers the first call's imports, which can be much slower than the body. Above 60 the engine may deliver its work again while it is still running."
         }
     },
     type: 'object',
@@ -1381,7 +1381,7 @@ export const NodeDef_OutputSchema = {
                 }
             ],
             title: 'Timeout',
-            description: "Seconds this node's code may run before it is stopped. Above 60 the engine may deliver its work again while it is still running."
+            description: "Seconds this node's code may run before it is stopped. This covers the first call's imports, which can be much slower than the body. Above 60 the engine may deliver its work again while it is still running."
         }
     },
     type: 'object',