货币体系: 全平台统一使用积分(POINT)替代人民币(CNY)

后端:
- 模型 currency 默认值 CNY→POINT (MerchantProduct/WalletAccount/FulfillmentOrder)
- DashboardStats TotalSales/TotalFees 从 float64 改为 int64，去掉 /100.0 转换
- 上游 OpenOrderQuery.Amount 从 float64 改为 int64，直接返回积分整数
- 钱包/商品创建时 currency 默认 POINT

前端:
- 去掉 centsToYuan/yuanToCents 转换函数，金额直接用整数积分
- money() 显示从 ¥X.XX 改为 X 积分
- 商品售价/成本表单字段名改回 price_amount/cost_amount，precision 改 0
- 钱包调整表单 amount_yuan→amount，precision 改 0
- 手续费固定金额表单 precision 改 0，label 改积分
- 币种选项 CNY→POINT
- Dashboard 成交金额/手续费 suffix 元→积分，去掉 precision

数据库:
- 新增迁移 004: currency 默认值 CNY→POINT，存量数据更新

frontend/src/pages/PlatformMerchants.tsx

@@ -70,7 +70,7 @@ export default function PlatformMerchants() {
         features: featureListToText(values.features),
         fee_type: values.fee_type,
         fee_rate_bp: Number(values.fee_rate_bp || 0),
-        fee_fixed_amount: yuanToCents(values.fee_fixed_yuan),
+        fee_fixed_amount: Number(values.fee_fixed_amount || 0),
       })
       message.success('商户已创建')
       setCreateOpen(false)
@@ -93,7 +93,7 @@ export default function PlatformMerchants() {
         features: featureListToText(values.features),
         fee_type: values.fee_type,
         fee_rate_bp: Number(values.fee_rate_bp || 0),
-        fee_fixed_amount: yuanToCents(values.fee_fixed_yuan),
+        fee_fixed_amount: Number(values.fee_fixed_amount || 0),
       })
       message.success('商户设置已更新')
       setSettingsOpen(false)
@@ -131,7 +131,7 @@ export default function PlatformMerchants() {
       width: 160,
       render: (_, r) =>
         r.fee_type === 'fixed'
-          ? `固定 ¥${centsToYuan(r.fee_fixed_amount).toFixed(2)}/单`
+          ? `固定 ${r.fee_fixed_amount} 积分/单`
           : `${(r.fee_rate_bp / 100).toFixed(2)}%`,
     },
     {
@@ -174,7 +174,7 @@ export default function PlatformMerchants() {
                 ...record,
                 features: featuresToList(record.features),
                 fee_type: record.fee_type || 'rate',
-                fee_fixed_yuan: centsToYuan(record.fee_fixed_amount),
+                fee_fixed_amount: record.fee_fixed_amount,
               })
               setSettingsOpen(true)
             }}
@@ -221,7 +221,7 @@ export default function PlatformMerchants() {
                 features: featureOptions.map((item) => item.value),
                 fee_type: 'rate',
                 fee_rate_bp: 0,
-                fee_fixed_yuan: 0,
+                fee_fixed_amount: 0,
               })
               setCreateOpen(true)
             }}
@@ -281,15 +281,15 @@ export default function PlatformMerchants() {
             <Select
               options={[
                 { value: 'rate', label: '按百分比（每单按订单金额比例收取）' },
-                { value: 'fixed', label: '按固定金额（每单固定手续费）' },
+                { value: 'fixed', label: '按固定金额（每单固定积分）' },
               ]}
             />
           </Form.Item>
           <Form.Item noStyle shouldUpdate={(prev, cur) => prev.fee_type !== cur.fee_type}>
             {({ getFieldValue }) =>
               getFieldValue('fee_type') === 'fixed' ? (
-                <Form.Item name="fee_fixed_yuan" label="每单固定手续费（元）" rules={[{ required: true }]}>
-                  <InputNumber min={0} precision={2} style={{ width: '100%' }} />
+                <Form.Item name="fee_fixed_amount" label="每单固定手续费（积分）" rules={[{ required: true }]}>
+                  <InputNumber min={0} precision={0} style={{ width: '100%' }} />
                 </Form.Item>
               ) : (
                 <Form.Item name="fee_rate_bp" label="手续费比例 BP（1BP=0.01%，如 250=2.5%）" rules={[{ required: true }]}>
@@ -324,15 +324,15 @@ export default function PlatformMerchants() {
             <Select
               options={[
                 { value: 'rate', label: '按百分比（每单按订单金额比例收取）' },
-                { value: 'fixed', label: '按固定金额（每单固定手续费）' },
+                { value: 'fixed', label: '按固定金额（每单固定积分）' },
               ]}
             />
           </Form.Item>
           <Form.Item noStyle shouldUpdate={(prev, cur) => prev.fee_type !== cur.fee_type}>
             {({ getFieldValue }) =>
               getFieldValue('fee_type') === 'fixed' ? (
-                <Form.Item name="fee_fixed_yuan" label="每单固定手续费（元）" rules={[{ required: true }]}>
-                  <InputNumber min={0} precision={2} style={{ width: '100%' }} />
+                <Form.Item name="fee_fixed_amount" label="每单固定手续费（积分）" rules={[{ required: true }]}>
+                  <InputNumber min={0} precision={0} style={{ width: '100%' }} />
                 </Form.Item>
               ) : (
                 <Form.Item name="fee_rate_bp" label="手续费比例 BP（1BP=0.01%，如 250=2.5%）" rules={[{ required: true }]}>
@@ -381,11 +381,3 @@ function featureListToText(features?: string[]) {
 function featureText(feature: string) {
   return featureOptions.find((item) => item.value === feature)?.label || feature
 }
-
-function yuanToCents(value?: number | null) {
-  return Math.round(Number(value || 0) * 100)
-}
-
-function centsToYuan(value?: number | null) {
-  return Number(((value || 0) / 100).toFixed(2))
-}