Report how long a run has been going, not just how long it took

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UytviPMJbXzD8P84nLvXcq

backend/tests/api/routes/test_observability.py

@@ -266,6 +266,21 @@ def test_runs_narrow_to_one_minute(
     assert [run["id"] for run in runs["data"]] == ["minute-in"]
 
 
+def test_a_running_cascade_reports_how_long_it_has_been_going(
+    client: TestClient, superuser_token_headers: dict[str, str], db: Session
+) -> None:
+    """The collector writes a duration at the end; until then, time since."""
+    started = datetime.now(UTC) - timedelta(seconds=30)
+    db.add(FlowRun(id="in-flight", flow=FLOW, started_at=started, status="running"))
+    db.commit()
+
+    runs = client.get(
+        f"{PREFIX}/runs", headers=superuser_token_headers, params={"status": "running"}
+    ).json()
+
+    assert runs["data"][0]["duration_ms"] >= 30_000
+
+
 def test_events_narrow_to_one_minute(
     client: TestClient, superuser_token_headers: dict[str, str], db: Session
 ) -> None: