Warm a worker before the node's clock starts, and refuse to delete a live flow

A node's timeout now covers its body only: the pool loads the source into the
worker it picked, off the node's budget, so imports that outlast the timeout no
longer make a node impossible to run. Draft checks compile without caching, so
saving does not evict what a busy node is serving calls from. Requests carry an
id the worker echoes and the pool checks, a reply is encoded once, and the
remote-exception cache is bounded.

DELETE /flows/{name} answers 409 while the flow has a running or queued run,
which is what was letting run_node rows outlive their run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01StpRc2C6au1WJ1EUU7fsfu

backend/tests/api/routes/test_flows.py

@@ -281,7 +281,9 @@ def test_delete_flow(
     client: TestClient, superuser_token_headers: dict[str, str], db: Session
 ) -> None:
     client.put(f"{PREFIX}/demo", headers=superuser_token_headers, json=a_flow())
-    db.add(Run(id="run-1", flow="demo", created_at=datetime.now(UTC)))
+    # Finished: a flow with a live run refuses to be deleted, which is its own
+    # test below.
+    db.add(Run(id="run-1", flow="demo", status="ok", created_at=datetime.now(UTC)))
     db.add(RunNode(run_id="run-1", node="sensor"))
     db.add(RunMetric(run_id="run-1", name="loss", step=-1))
     db.add(RunArtifact(run_id="run-1", name="model.pt"))
@@ -301,6 +303,24 @@ def test_delete_flow(
         assert db.exec(select(func.count()).select_from(model)).one() == 0
 
 
+def test_delete_flow_is_refused_while_a_run_is_live(
+    client: TestClient, superuser_token_headers: dict[str, str], db: Session
+) -> None:
+    client.put(f"{PREFIX}/demo", headers=superuser_token_headers, json=a_flow())
+    db.add(
+        Run(id="run-live", flow="demo", status="running", created_at=datetime.now(UTC))
+    )
+    db.commit()
+
+    response = client.delete(f"{PREFIX}/demo", headers=superuser_token_headers)
+    assert response.status_code == 409
+    assert "run-live" in response.json()["detail"]
+    # Its driver is still writing node rows against that id, so nothing may go.
+    assert (
+        client.get(f"{PREFIX}/demo", headers=superuser_token_headers).status_code == 200
+    )
+
+
 def test_node_types_are_listed(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None: