Let a teardown's cancellation through, and reap the workers it leaves

Node stop paths cancelled their background task and then caught
CancelledError around the await, which swallows a cancellation aimed at
the caller — the trap Supervisor._cancel already documents. One shared
Node._cancel_task now waits the way the supervisor does; mqtt's publisher
and subscription and delay's cron call it.

The api container also collected zombie python workers: orphaned when
--reload replaces the process holding their handle, they reparent onto a
PID 1 that reaps nothing but its own. `init: true` on the backend service.

backend/tests/flow/test_node_teardown.py

@@ -0,0 +1,31 @@
+"""Tearing a node down must not swallow a cancellation meant for the caller."""
+
+import asyncio
+
+import pytest
+
+from fluksio.flow.nodes import DelayNode
+
+
+def test_stop_cron_lets_the_callers_cancellation_through():
+    async def stubborn() -> None:
+        """A loop whose shutdown does not answer the first cancellation."""
+        try:
+            await asyncio.sleep(3600)
+        except asyncio.CancelledError:
+            await asyncio.sleep(3600)
+
+    async def scenario() -> None:
+        node = DelayNode(params={"cron": "* * * * *"})
+        node._stop_cron = asyncio.Event()
+        node._cron_task = asyncio.create_task(stubborn())
+
+        stopping = asyncio.create_task(node.stop_cron())
+        await asyncio.sleep(0.05)  # let it reach the await on the cron task
+        stopping.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await stopping
+
+        node._cron_task.cancel()
+
+    asyncio.run(scenario())