Health: a flow that cannot run says so, and the brain marks which neurons

A dependency loop is flagged on the canvas and was invisible everywhere else:
/observability/summary answered "ok" with an empty problems list while the
published flow could not run at all. It now reports the flows validation
blocks, and the brain graph carries the reason on each neuron the issue names
so the view built to find broken wiring can show it.

Node errors stay counted once, as the nodes that failed to load, and an
advisory like an unauthenticated webhook marks nothing — it is worth saying,
but the flow still runs.

frontend/src/client/schemas.gen.ts

@@ -265,6 +265,18 @@ export const BrainNodeSchema = {
             },
             type: 'array',
             title: 'Flows'
+        },
+        issue: {
+            anyOf: [
+                {
+                    type: 'string'
+                },
+                {
+                    type: 'null'
+                }
+            ],
+            title: 'Issue',
+            description: 'Why this neuron cannot run, if validation found something. A failure the engine hits while running arrives over the socket instead; this is the part that is already true before anything fires, and so has to travel with the graph.'
         }
     },
     type: 'object',

frontend/src/client/types.gen.ts

@@ -109,6 +109,10 @@ export type BrainNode = {
     kind: string;
     members?: Array<(string)>;
     flows?: Array<(string)>;
+    /**
+     * Why this neuron cannot run, if validation found something. A failure the engine hits while running arrives over the socket instead; this is the part that is already true before anything fires, and so has to travel with the graph.
+     */
+    issue?: (string | null);
 };
 
 /**