Backend: real alert test results, state cleanup on delete/rename, node trigger errors, queue and collector fixes

`AlertManager.send` swallowed every delivery failure, so the alerts screen's
Test button answered 200 whatever happened — the one thing it exists for. It
takes `raise_on_error` now, which only the test route passes; the per-channel
loop keeps the swallow, because one dead channel must not stop the others
hearing about the same fault. A refused delivery answers 502 with whatever the
sender said.

Renaming a flow left its values under the old name for good: the delete path
already swept them, the rename path never did. It calls the same `forget_flow`,
which covers the messages and the `__ts__`/`__version__`/`__history__`
bookkeeping keyed by message name. Cleanup, not migration — they repopulate
under the new name on the next run.

Triggering a node by hand ran `Node.__call__` with nothing catching it, so a
node that raised produced a 500 and a stack trace in the server log, and
nothing at all on the canvas. `Pipeline.publish_error` is the reporting half of
`_execute_node` lifted out; both paths go through it, so a manual failure now
reads the same on the canvas and in the metrics as a queued one. The route
answers 400 with the node's error.

`MemoryWorkQueue.stats()` counts claimed-but-unacknowledged work rather than
reporting zero, so the health tile means something without Redis. The metrics
collector's held tracebacks are capped at `DETAIL_CAP` and swept on the same
`RUN_STALE_S` cutoff the open runs use, instead of one untruncated traceback
per node kept for the life of the process — a traceback still survives the
flush between the log and the failure it belongs to.

`GET /observability/events` takes `since`/`until`, the window `/runs` already
took, so a failures list can cover the span the charts beside it are drawn from.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XC2jX6Hdj7pxGGKzBTrbqB

backend/tests/api/routes/test_flows.py

@@ -357,3 +357,69 @@ def test_rename_rejects_an_invalid_name(
         json={"new_name": "Not A Flow Name"},
     )
     assert response.status_code == 400
+
+
+def test_a_node_that_raises_answers_with_its_error(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """A manual run fails the way a scheduled one does, not as a 500."""
+    saved = client.put(
+        f"{PREFIX}/failing", headers=superuser_token_headers, json=a_flow("failing")
+    ).json()
+    client.put(
+        f"{PREFIX}/failing/nodes/sensor/source",
+        headers=superuser_token_headers,
+        json={"code": BROKEN_NODE},
+    )
+    client.post(
+        f"{PREFIX}/failing/publish",
+        headers=superuser_token_headers,
+        json={"version": saved["definition"]["version"]},
+    )
+
+    response = client.post(
+        f"{PREFIX}/failing/nodes/sensor/trigger",
+        headers=superuser_token_headers,
+        json={"values": {}},
+    )
+
+    assert response.status_code == 400
+    assert "boom" in response.json()["detail"]
+
+    client.delete(f"{PREFIX}/failing", headers=superuser_token_headers)
+
+
+def test_renaming_a_flow_leaves_nothing_under_the_old_name(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    saved = client.put(
+        f"{PREFIX}/movable", headers=superuser_token_headers, json=a_flow("movable")
+    ).json()
+    client.put(
+        f"{PREFIX}/movable/nodes/sensor/source",
+        headers=superuser_token_headers,
+        json={"code": WORKING_NODE},
+    )
+    client.post(
+        f"{PREFIX}/movable/publish",
+        headers=superuser_token_headers,
+        json={"version": saved["definition"]["version"]},
+    )
+    client.post(
+        f"{PREFIX}/movable/run", headers=superuser_token_headers, json={"inputs": {}}
+    )
+
+    state = client.app.state.flow_controller.state
+    assert [key for key in state.keys() if "movable." in key]
+
+    client.post(
+        f"{PREFIX}/movable/rename",
+        headers=superuser_token_headers,
+        json={"new_name": "moved"},
+    )
+
+    # The values repopulate under the new name on the next run; what the old
+    # name left behind would sit there for good.
+    assert [key for key in state.keys() if "movable." in key] == []
+
+    client.delete(f"{PREFIX}/moved", headers=superuser_token_headers)