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

openapi/openapi.yaml

@@ -678,9 +678,24 @@ paths:
       description: 'Return time-range readings for a device.
 
 
-        Results are ordered by ``recorded_at`` ascending and capped by ``limit``
+        When the window contains more rows than ``limit``, the **most recent** N rows
 
-        (max {_READINGS_LIMIT_MAX}) to prevent accidental full-table exports.
+        are returned (``ORDER BY recorded_at DESC LIMIT n``), then reversed to
+
+        ascending order before being sent to the client.  This ensures that for long
+
+        time-range requests (e.g. 6 h / 24 h) the caller always sees the latest data
+
+        rather than the oldest segment of the window.
+
+
+        When the window has fewer rows than ``limit`` the full window is returned
+        in
+
+        ascending order — behaviour is identical to a plain ascending query.
+
+
+        The response schema is unchanged: items are always ``recorded_at`` ascending.
 
 
         The query uses the ``(device_id, recorded_at)`` composite index for