Prevent terminal auth spinner deadlock

tests/test_web_observability.py

@@ -775,6 +775,43 @@ def test_city_detail_batch_chart_scope_returns_only_chart_fields(monkeypatch):
     assert "ai_analysis" not in detail
 
 
+def test_chart_detail_payload_avoids_threadpool_queue_and_reuses_short_cache(monkeypatch):
+    import asyncio
+
+    build_calls = 0
+
+    async def fail_threadpool(*_args, **_kwargs):
+        raise AssertionError("chart payload should not wait behind the shared threadpool")
+
+    def build_chart_detail(data, resolution):
+        nonlocal build_calls
+        build_calls += 1
+        return {
+            "city": data["city"],
+            "resolution": resolution,
+            "hourly": data["hourly"],
+        }
+
+    city_api._CITY_CHART_DETAIL_PAYLOAD_CACHE.clear()
+    city_api._CITY_CHART_DETAIL_PAYLOAD_CACHE_TS.clear()
+    monkeypatch.setenv("POLYWEATHER_CITY_DETAIL_PAYLOAD_CACHE_TTL_SEC", "20")
+    monkeypatch.setattr(city_api, "run_in_threadpool", fail_threadpool)
+    monkeypatch.setattr(city_api.legacy_routes, "_build_city_chart_detail_payload", build_chart_detail)
+
+    data = {
+        "city": "paris",
+        "updated_at": "2026-05-30T15:00:00Z",
+        "hourly": {"times": ["2026-05-30T15:00:00Z"], "temps": [20.0]},
+    }
+
+    first = asyncio.run(city_api._build_city_chart_detail_payload(data, "10m"))
+    second = asyncio.run(city_api._build_city_chart_detail_payload(data, "10m"))
+
+    assert first == second
+    assert first["resolution"] == "10m"
+    assert build_calls == 1
+
+
 def test_city_detail_batch_endpoint_limits_backend_concurrency(monkeypatch):
     import asyncio