Let a pinned minute read runs from the whole day

/observability/runs gains since/until, so the throughput chart's pin asks
the server for its minute instead of filtering a fixed recent list. This
engine writes ~60 runs a minute, so any minute but the newest read empty.

since is inclusive and until exclusive, matching the minute buckets the
charts are drawn from. Hover stays the client-side preview it was:
scrubbing a day would otherwise be a request per minute rested on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

backend/tests/api/routes/test_observability.py

@@ -102,3 +102,35 @@ def test_the_history_reads_back(
     dead = client.get(f"{PREFIX}/dead-letter", headers=superuser_token_headers)
     assert dead.status_code == 200
     assert isinstance(dead.json(), list)
+
+
+def test_runs_narrow_to_one_minute(
+    client: TestClient, superuser_token_headers: dict[str, str], db: Session
+) -> None:
+    """A minute picked off a chart reaches past what the recent list holds."""
+    minute = datetime.now(timezone.utc).replace(second=0, microsecond=0) - timedelta(
+        hours=3
+    )
+    db.add(FlowRun(id="minute-in", flow=FLOW, started_at=minute, status="ok"))
+    db.add(
+        FlowRun(
+            id="minute-after",
+            flow=FLOW,
+            started_at=minute + timedelta(minutes=1),
+            status="ok",
+        )
+    )
+    db.commit()
+
+    runs = client.get(
+        f"{PREFIX}/runs",
+        headers=superuser_token_headers,
+        params={
+            "since": minute.isoformat(),
+            "until": (minute + timedelta(minutes=1)).isoformat(),
+        },
+    ).json()
+
+    # 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"]