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/tests/api/routes/test_observability.py

@@ -127,6 +127,37 @@ def test_a_flow_that_cannot_run_makes_the_summary_degraded(
     assert not any("hooky" in problem for problem in body["problems"])
 
 
+def test_a_down_node_makes_the_summary_degraded(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """A connector that cannot reach its device is not a flow that cannot run.
+
+    It is counted on its own, so the flow keeps running and "invalid" stays
+    about validation.
+    """
+    from fluksio.flow.controller import LoadedNode
+
+    controller = client.app.state.flow_controller
+    before = controller.loaded
+    controller.loaded = {
+        "house.owm": LoadedNode(
+            id="house.owm",
+            flow="house",
+            health="down",
+            health_detail="ConnectionError: name resolution failed",
+        )
+    }
+    try:
+        body = client.get(f"{PREFIX}/summary", headers=superuser_token_headers).json()
+    finally:
+        controller.loaded = before
+
+    assert body["status"] == "degraded"
+    assert body["nodes"]["unhealthy"] == 1
+    assert any("down" in problem for problem in body["problems"])
+    assert body["flows"]["invalid"] == 0
+
+
 def test_the_history_reads_back(
     client: TestClient, superuser_token_headers: dict[str, str], db: Session
 ) -> None: