Surface a failing connector poll as node health and a flow issue

The poll loop remembered what it read rather than what it published, so a
value the node could not publish counted as said: the next poll skipped it,
succeeded, and health went back to ok with the port still dark. Remember it
only after inject returns, and report ok last.

A node reporting itself down is now derived into its flow's issues on read
and counted on the health summary, so the canvas marks it and Home says so.
Being down does not stop the flow, and the issue clears by itself when the
node reports well again. The repeating poll warning is logged once per
outage rather than once per tick.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

backend/fluksio/flow/controller.py

@@ -1224,8 +1224,31 @@ class FlowController:
             }
         )
 
+    def _health_issues(self, flow: str | None = None) -> list[ValidationIssue]:
+        """Nodes that are running but not working, as issues on their flow.
+
+        Not part of `self.issues`: that list is what a build found, and this is
+        what is happening now. Derived on read, so a node reporting itself well
+        again clears it with nothing to remember.
+        """
+        return [
+            ValidationIssue(
+                code="node_unhealthy",
+                message=(
+                    f"Node '{entry.id.rpartition('.')[2]}' is down: "
+                    f"{entry.health_detail or 'no detail given'}"
+                ),
+                flow=entry.flow,
+                node=entry.id,
+            )
+            for entry in self.loaded.values()
+            if entry.health == "down" and (flow is None or entry.flow == flow)
+        ]
+
     def flow_issues(self, flow: str) -> list[ValidationIssue]:
-        return [issue for issue in self.issues if not issue.flow or issue.flow == flow]
+        return [
+            issue for issue in self.issues if not issue.flow or issue.flow == flow
+        ] + self._health_issues(flow)
 
     def preview(self, name: str) -> Preview:
         """Build a flow's unpublished draft without deploying it.