Add support for inverter current caps in site configuration

- Introduced `InverterModbusCurrentCapsBody` model for updating max charge and discharge currents.
- Updated SQL queries to utilize `COALESCE` for effective current limits.
- Modified relevant tests to reflect changes in battery current handling.
- Added new SQL migration for `deye_register_max_current_a` columns in the database.

backend/tests/test_control_exporter_tou.py

@@ -36,7 +36,7 @@ def _inv(*, min_soc: int | None = 12, reserve_soc: int | None = 20) -> InverterC
 class DeyeTouParamsTests(unittest.TestCase):
     def test_sell_uses_reserve_soc(self) -> None:
         sp = ControlSetpoints(
-            battery_w=0,
+            battery_w=-600,
             grid_export_limit=5000,
             ev1_current_a=0,
             ev2_current_a=0,