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/flow/test_queue.py

@@ -60,6 +60,20 @@ def test_a_deleted_flow_leaves_nothing_parked():
     assert queue.unpark("gone") == []
 
 
+def test_claimed_work_counts_as_in_flight_until_it_is_acknowledged():
+    """The health tile's "in flight" reads zero without this."""
+    queue = MemoryWorkQueue()
+    queue.add(WorkItem(kind="cascade", node="f.n", flow="f"))
+
+    assert queue.stats()["pending"] == 0
+
+    (item,) = queue.claim(1, 10)
+    assert queue.stats()["pending"] == 1
+
+    queue.ack(item)
+    assert queue.stats()["pending"] == 0
+
+
 def _pipeline_with_a_consumer() -> tuple[Pipeline, Node, MemoryState, list]:
     """A source whose message a consumer records."""
     seen: list[float] = []