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/fluksio/flow/nodes/mqtt.py

@@ -461,11 +461,7 @@ class MqttNode(Node):
         if self._publish_queue is None:
             return
         if self._publisher_task is not None:
-            self._publisher_task.cancel()
-            try:
-                await self._publisher_task
-            except (asyncio.CancelledError, Exception):  # noqa: B014 - shutting down
-                pass
+            await self._cancel_task(self._publisher_task)
         self._publisher_task = None
         self._publish_queue = None
         self._loop = None
@@ -515,11 +511,7 @@ class MqttNode(Node):
         self._stop_event.set()
 
         if self._subscription_task is not None:
-            self._subscription_task.cancel()
-            try:
-                await self._subscription_task
-            except asyncio.CancelledError:
-                pass
+            await self._cancel_task(self._subscription_task)
 
         self._subscription_task = None
         self._stop_event = None