Supervise the loops a flow starts, and give up loudly

A node's subscription, schedule or poll loop was a bare asyncio task: one
that raised outside its own retry handling was simply gone, and the node
went on being listed as running while nothing listened any more.

Those loops now run under a supervisor that restarts them with a growing
delay and quarantines a flow that burns through five restarts in five
minutes — a flow crash-looping every second is worse than one that is
visibly stopped, and the dashboard can now say which.

The MQTT subscription loses its private five-second reconnect in the
process: one backoff policy per socket, and it belongs to the supervisor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011LF61rxW1FG5YCD2J9YqjY

backend/app/api/routes/flows.py

@@ -172,6 +172,7 @@ def read_flows(controller: FlowControllerDep) -> Any:
                 has_draft=controller.store.has_draft(name),
                 enabled=controller.is_enabled(name),
                 paused=controller.is_paused(name),
+                quarantined=controller.is_quarantined(name),
             )
         )
     return FlowsPublic(data=summaries, count=len(summaries))