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_observability.py

@@ -134,3 +134,35 @@ def test_runs_narrow_to_one_minute(
     # The upper bound is exclusive, so the run starting the next minute is not
     # in this one.
     assert [run["id"] for run in runs] == ["minute-in"]
+
+
+def test_events_narrow_to_one_minute(
+    client: TestClient, superuser_token_headers: dict[str, str], db: Session
+) -> None:
+    """The failures list reaches as far back as the charts beside it."""
+    minute = datetime.now(timezone.utc).replace(second=0, microsecond=0) - timedelta(
+        hours=4
+    )
+    db.add(EngineEvent(ts=minute, type="node_error", flow=FLOW, detail="minute-in"))
+    db.add(
+        EngineEvent(
+            ts=minute + timedelta(minutes=1),
+            type="node_error",
+            flow=FLOW,
+            detail="minute-after",
+        )
+    )
+    db.commit()
+
+    events = client.get(
+        f"{PREFIX}/events",
+        headers=superuser_token_headers,
+        params={
+            "flow": FLOW,
+            "since": minute.isoformat(),
+            "until": (minute + timedelta(minutes=1)).isoformat(),
+        },
+    ).json()
+
+    # The upper bound is exclusive, so the failure a minute later is not in it.
+    assert [event["detail"] for event in events] == ["minute-in"]