M5-T07: add latest-reading cards and Recharts trend charts; readings return most-recent rows

tests/test_api_modbus.py

@@ -548,6 +548,7 @@ def test_readings_time_window_filters_correctly(modbus_client):
 
 
 def test_readings_limit_applied(modbus_client):
+    """When limit < total rows, exactly *limit* items are returned."""
     client, engine = modbus_client
     _login(client)
     device = _make_device(engine)
@@ -564,6 +565,49 @@ def test_readings_limit_applied(modbus_client):
     assert len(resp.json()["items"]) == 3
 
 
+def test_readings_limit_returns_most_recent_rows_ascending(modbus_client):
+    """When window rows > limit, the MOST RECENT limit rows are returned, ascending.
+
+    This is the regression test for REWORK-1: previously the endpoint returned
+    the oldest N rows (ASC + LIMIT), causing 6h/24h charts to show stale data.
+    Now it returns the most recent N rows (DESC + LIMIT, then reversed to ASC).
+    """
+    client, engine = modbus_client
+    _login(client)
+    device = _make_device(engine)
+
+    now = datetime.now(UTC)
+    # Insert 5 readings spread over the last 5 minutes (newest first, but order shouldn't matter).
+    timestamps = [now - timedelta(minutes=i) for i in range(5)]  # t0=now, t1=now-1m, ..., t4=now-4m
+    for idx, ts in enumerate(timestamps):
+        _make_reading(engine, device.id, ts, {"voltage": float(200 + idx)})
+    # Chronologically: t4 < t3 < t2 < t1 < t0
+    # Voltages mapped:  204    203   202   201   200
+
+    # Request only the most recent 3 rows (limit=3).
+    resp = client.get(
+        f"/api/modbus/devices/{device.uuid}/readings",
+        params={"limit": 3},
+    )
+    assert resp.status_code == 200
+    items = resp.json()["items"]
+    assert len(items) == 3
+
+    # Response must be in ascending recorded_at order.
+    recorded_ats = [item["recorded_at"] for item in items]
+    assert recorded_ats == sorted(recorded_ats), "Items must be ascending by recorded_at"
+
+    # The 3 most recent are timestamps[0], [1], [2] → voltages 200, 201, 202.
+    voltages = [item["payload"]["voltage"] for item in items]
+    assert voltages == [202.0, 201.0, 200.0], (
+        f"Expected most recent 3 rows (voltages 200/201/202 ascending), got {voltages}"
+    )
+
+    # The oldest row (voltage 204.0, at t4=now-4m) must NOT be in the response.
+    assert 204.0 not in voltages, "Oldest row must not appear when limit truncates the window"
+    assert 203.0 not in voltages, "Second-oldest row must not appear when limit truncates"
+
+
 def test_readings_limit_exceeds_max_returns_422(modbus_client):
     """A limit > 5000 must be rejected by FastAPI query validation (422)."""
     client, engine = modbus_client