feat: 支持章节通过 mid 进行访问，优化阅读跳转逻辑。新增章节数据结构，包含章节的 mid 信息，提升用户体验。更新 API 以支持通过 mid 查询章节内容，确保兼容性与灵活性。

soul-api/internal/handler/user.go

@@ -268,12 +268,21 @@ func UserPurchaseStatus(c *gin.Context) {
 		c.JSON(http.StatusNotFound, gin.H{"success": false, "error": "用户不存在"})
 		return
 	}
-	var orderRows []struct{ ProductID string }
-	db.Raw("SELECT DISTINCT product_id FROM orders WHERE user_id = ? AND status = ? AND product_type = ?", userId, "paid", "section").Scan(&orderRows)
+	var orderRows []struct {
+		ProductID string
+		MID       int
+	}
+	db.Raw(`SELECT DISTINCT o.product_id, c.mid FROM orders o 
+		LEFT JOIN chapters c ON c.id = o.product_id 
+		WHERE o.user_id = ? AND o.status = ? AND o.product_type = ?`, userId, "paid", "section").Scan(&orderRows)
 	purchasedSections := make([]string, 0, len(orderRows))
+	sectionMidMap := make(map[string]int)
 	for _, r := range orderRows {
 		if r.ProductID != "" {
 			purchasedSections = append(purchasedSections, r.ProductID)
+			if r.MID > 0 {
+				sectionMidMap[r.ProductID] = r.MID
+			}
 		}
 	}
 	// 匹配次数配额：纯计算（订单 + match_records）
@@ -290,6 +299,7 @@ func UserPurchaseStatus(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"success": true, "data": gin.H{
 		"hasFullBook":       user.HasFullBook != nil && *user.HasFullBook,
 		"purchasedSections": purchasedSections,
+		"sectionMidMap":     sectionMidMap,
 		"purchasedCount":    len(purchasedSections),
 		"matchCount":        matchQuota.PurchasedTotal,
 		"matchQuota": gin.H{