feat(backend): license_purchases 增 amount_minor/currency/pay_url/renewed_to 列 + 存量回填（pay v2 金额口径）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/service/pay.go

@@ -368,6 +368,21 @@ func entitle(tx *gorm.DB, shopID uint64, plan payPlan) error {
 	}).Error
 }
 
+// BackfillPurchaseAmountMinor 启动回填：v1 存量购买单 amount("2999.00") → amount_minor(299900)+CNY。
+// 幂等：只处理 amount_minor=0 且 amount 非空的行（参照 backfillPinyin 先例）。
+func BackfillPurchaseAmountMinor(db *gorm.DB) {
+	var rows []model.LicensePurchase
+	if err := db.Where("amount_minor = 0 AND amount <> ''").Find(&rows).Error; err != nil {
+		return
+	}
+	for _, p := range rows {
+		if c, err := toCents(p.Amount); err == nil && c > 0 {
+			db.Model(&model.LicensePurchase{}).Where("id = ?", p.ID).
+				Updates(map[string]any{"amount_minor": c, "currency": "CNY"})
+		}
+	}
+}
+
 // amountEqual 金额按分归一比较（"2999.00" == "2999" == "2999.0"）。
 func amountEqual(a, b string) bool {
 	ca, ea := toCents(a)