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.

db/migration/V044__deye_register_max_current_a.sql

@@ -4,6 +4,6 @@ ALTER TABLE ems.asset_inverter
     ADD COLUMN IF NOT EXISTS deye_register_max_discharge_a INT NULL;
 
 COMMENT ON COLUMN ems.asset_inverter.deye_register_max_charge_a IS
-    'Optional cap for holding reg 108 (A); NULL = use only LEAST(W)/51.2 derived max.';
+    'Optional A for reg 108; EMS uses COALESCE(this, FLOOR(LEAST(W)/51.2)) in _load_inverter_config.';
 COMMENT ON COLUMN ems.asset_inverter.deye_register_max_discharge_a IS
-    'Optional cap for holding reg 109 (A); NULL = use only derived max.';
+    'Optional A for reg 109; EMS uses COALESCE(this, FLOOR(LEAST(W)/51.2)) in _load_inverter_config.';