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.

frontend/src/client/schemas.gen.ts

@@ -364,6 +364,11 @@ export const FlowSummarySchema = {
             type: 'boolean',
             title: 'Paused',
             default: false
+        },
+        quarantined: {
+            type: 'boolean',
+            title: 'Quarantined',
+            default: false
         }
     },
     type: 'object',

frontend/src/client/types.gen.ts

@@ -101,6 +101,7 @@ export type FlowSummary = {
     has_draft?: boolean;
     enabled?: boolean;
     paused?: boolean;
+    quarantined?: boolean;
 };
 
 /**