Refuse a port the function cannot take, and read a failing node as degraded

A python node's ports and settings arrive as keyword arguments, so a declared
name its `process` does not take was a TypeError on every call — and a node
that loads fine and fails every time it runs is the quiet kind of broken: the
hosted demo did it 720 times an hour for two days and the health badge read
ok throughout. `_build_node` now reads a written body with `ast` and refuses
the mismatch at load, so the node is an error on the canvas and an issue on
publish. Skipped for `**kwargs`, a decorated or absent `process`, and the
template a new node opens with. The SDK's generated shim always takes
`**settings`, so synced flows are untouched.

`/observability/summary` names a node that has failed in the last fifteen
minutes and reads degraded while it does, which is what would have made the
badge amber. `nodes.failing` carries the count.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SkgNtaR6JspnHBFFP6crZj

backend/tests/api/routes/test_observability.py

@@ -447,3 +447,38 @@ def test_a_timeseries_window_can_be_named(
     assert response.status_code == 200
     # Two of the four minutes: `since` inclusive, `until` exclusive.
     assert sum(point["executions"] for point in response.json()) == 2
+
+
+def test_a_node_failing_on_every_call_makes_the_summary_degraded(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """A node that loads and then raises each time it runs is not in `error`.
+
+    It is named while its last failure is recent, and drops out once it is not.
+    """
+    import time
+
+    from fluksio.flow.controller import LoadedNode
+
+    controller = client.app.state.flow_controller
+    before = controller.loaded
+    controller.loaded = {
+        "house.calc": LoadedNode(
+            id="house.calc", flow="house", last_error="boom", last_error_ts=time.time()
+        ),
+        "house.old": LoadedNode(
+            id="house.old",
+            flow="house",
+            last_error="boom",
+            last_error_ts=time.time() - 3600,
+        ),
+    }
+    try:
+        body = client.get(f"{PREFIX}/summary", headers=superuser_token_headers).json()
+    finally:
+        controller.loaded = before
+
+    assert body["status"] == "degraded"
+    assert body["nodes"]["failing"] == 1
+    (problem,) = [p for p in body["problems"] if "failing" in p]
+    assert "house.calc" in problem and "house.old" not in problem