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/fluksio/api/routes/flows.py

@@ -454,6 +454,19 @@ async def delete_flow(
     name: str, controller: FlowControllerDep, user: CurrentUser, session: SessionDep
 ) -> Any:
     """Delete a flow and everything in it."""
+    live = session.exec(
+        select(Run.id).where(
+            col(Run.flow) == name, col(Run.status).in_(("running", "queued"))
+        )
+    ).first()
+    if live is not None:
+        raise HTTPException(
+            status_code=409,
+            detail=(
+                f"Flow '{name}' has a run in progress ({live}). Cancel it, or "
+                "wait for it to finish, before deleting the flow."
+            ),
+        )
     try:
         await run_in_threadpool(controller.store.delete_flow, name)
     except FlowNotFound: