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/test_metrics.py

@@ -11,7 +11,7 @@ from datetime import datetime, timezone
 from sqlmodel import Session, select
 
 from app.flow.events import EventBus
-from app.flow.metrics import MetricsCollector
+from app.flow.metrics import DETAIL_CAP, RUN_STALE_S, MetricsCollector
 from app.models import EngineEvent, FlowRun, MetricBucket
 
 FLOW = "metrics-test"
@@ -197,3 +197,30 @@ def test_a_run_that_did_not_fail_reads_ok(db: Session) -> None:
 
     run = db.exec(select(FlowRun).where(FlowRun.id == "manual-abc")).one()
     assert (run.status, run.source) == ("ok", "manual")
+
+
+def test_a_traceback_no_failure_ever_claims_is_dropped() -> None:
+    """One entry per node that ever errored would else last the process out."""
+    node = f"{FLOW}.stale"
+    collector = MetricsCollector(EventBus())
+    collector.handle(
+        {
+            "type": "node_log",
+            "flow": FLOW,
+            "node": node,
+            "level": "error",
+            "text": "x" * (DETAIL_CAP * 2),
+            "ts": time.time() - RUN_STALE_S - 1,
+        }
+    )
+
+    # Untruncated, one of these holds a whole run of a chatty node.
+    assert len(collector._tracebacks[(FLOW, node)][1]) == DETAIL_CAP
+
+    # Something to write, so the flush does not stop at its early return.
+    collector.handle(
+        {"type": "node_executed", "flow": FLOW, "node": node, "ts": time.time()}
+    )
+    asyncio.run(collector.flush())
+
+    assert collector._tracebacks == {}