feat: add configurable payment amount precision

- add system.amount_precision setting with 2-6 validation

src/controller/admin/settings_controller.go

@@ -1,6 +1,8 @@
 package admin
 
 import (
+	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/GMWalletApp/epusdt/model/dao"
@@ -48,6 +50,7 @@ import (
 //
 //   - group=system:
 //     system.order_expiration_time (int) — order expiry in minutes
+//     system.amount_precision      (int) — payment amount precision, 2-6 decimals (default 2)
 type SettingUpsertItem struct {
 	Group string `json:"group" enums:"brand,rate,system,epay,okpay" example:"epay"`
 	Key   string `json:"key" example:"epay.default_network"`
@@ -91,7 +94,7 @@ func (c *BaseAdminController) ListSettings(ctx echo.Context) error {
 // @Description  okpay group keys: okpay.enabled, okpay.shop_id, okpay.shop_token, okpay.api_url, okpay.callback_url, okpay.return_url, okpay.timeout_seconds, okpay.allow_tokens.
 // @Description  rate group keys: rate.forced_usdt_rate (>0 overrides USDT/CNY; <=0 uses rate.api_url), rate.api_url, rate.adjust_percent, rate.okx_c2c_enabled.
 // @Description  brand group keys: brand.checkout_name, brand.logo_url, brand.site_title, brand.success_copy, brand.support_url, brand.background_color, brand.background_image_url. Legacy aliases brand.site_name, brand.page_title and brand.pay_success_text are also supported.
-// @Description  system group keys: system.order_expiration_time.
+// @Description  system group keys: system.order_expiration_time, system.amount_precision (int, 2-6, default 2).
 // @Tags         Admin Settings
 // @Security     AdminJWT
 // @Accept       json
@@ -120,6 +123,14 @@ func (c *BaseAdminController) UpsertSettings(ctx echo.Context) error {
 			out = append(out, result{Key: item.Key, OK: false, Error: "key required"})
 			continue
 		}
+		if err := validateSettingItem(item.Group, key, item.Value); err != nil {
+			out = append(out, result{Key: key, OK: false, Error: err.Error()})
+			continue
+		}
+		if key == mdb.SettingKeyAmountPrecision {
+			item.Group = mdb.SettingGroupSystem
+			item.Type = mdb.SettingTypeInt
+		}
 		if err := data.SetSetting(item.Group, key, item.Value, item.Type); err != nil {
 			out = append(out, result{Key: key, OK: false, Error: err.Error()})
 			continue
@@ -148,6 +159,23 @@ func (c *BaseAdminController) UpsertSettings(ctx echo.Context) error {
 	return c.SucJson(ctx, out)
 }
 
+func validateSettingItem(group, key, value string) error {
+	switch key {
+	case mdb.SettingKeyAmountPrecision:
+		if strings.ToLower(strings.TrimSpace(group)) != mdb.SettingGroupSystem {
+			return fmt.Errorf("%s must use group %s", key, mdb.SettingGroupSystem)
+		}
+		precision, err := strconv.Atoi(strings.TrimSpace(value))
+		if err != nil {
+			return fmt.Errorf("%s must be an integer", key)
+		}
+		if precision < data.MinAmountPrecision || precision > data.MaxAmountPrecision {
+			return fmt.Errorf("%s must be between %d and %d", key, data.MinAmountPrecision, data.MaxAmountPrecision)
+		}
+	}
+	return nil
+}
+
 // DeleteSetting removes one row. The next read of that key will fall
 // back to the hardcoded default (see settings_data.GetSetting*).
 // @Summary      Delete setting