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

app/api/routes/api/modbus.py

@@ -304,8 +304,16 @@ def get_readings(
 ) -> ModbusReadingsResponse:
     """Return time-range readings for a device.
 
-    Results are ordered by ``recorded_at`` ascending and capped by ``limit``
-    (max {_READINGS_LIMIT_MAX}) to prevent accidental full-table exports.
+    When the window contains more rows than ``limit``, the **most recent** N rows
+    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
     efficient time-window scans.
@@ -320,7 +328,7 @@ def get_readings(
     stmt = (
         select(ModbusReading)
         .where(ModbusReading.device_id == device.id)
-        .order_by(ModbusReading.recorded_at)
+        .order_by(ModbusReading.recorded_at.desc())
         .limit(limit)
     )
 
@@ -329,7 +337,8 @@ def get_readings(
     if end is not None:
         stmt = stmt.where(ModbusReading.recorded_at <= end)
 
-    rows = db.execute(stmt).scalars().all()
+    # Fetch most-recent N rows, then reverse to restore ascending order for the response.
+    rows = list(reversed(db.execute(stmt).scalars().all()))
     items = [ModbusReadingResponse(recorded_at=r.recorded_at, payload=r.payload) for r in rows]
     return ModbusReadingsResponse(items=items)