init: 初始化 AssetX 项目仓库

包含 webapp（Next.js 用户端）、webapp-back（Go 后端）、
antdesign（管理后台）、landingpage（营销落地页）、
数据库 SQL 和配置文件。

webapp-back/models/nulltime.go

@@ -0,0 +1,67 @@
+package models
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"strings"
+	"time"
+)
+
+// NullTime wraps *time.Time with custom JSON and GORM handling:
+//   - JSON marshal:   nil → "1970-01-01T00:00:00Z", non-nil → RFC3339
+//   - JSON unmarshal: null / "" / "1970-01-01..." → nil (stored as NULL in DB)
+//   - GORM Scan/Value: nil ↔ NULL column
+type NullTime struct {
+	Time *time.Time
+}
+
+func (nt NullTime) MarshalJSON() ([]byte, error) {
+	if nt.Time == nil {
+		return []byte(`"1970-01-01T00:00:00Z"`), nil
+	}
+	return json.Marshal(nt.Time.Format(time.RFC3339))
+}
+
+func (nt *NullTime) UnmarshalJSON(data []byte) error {
+	if string(data) == "null" {
+		nt.Time = nil
+		return nil
+	}
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+	if s == "" || strings.HasPrefix(s, "1970-01-01") {
+		nt.Time = nil
+		return nil
+	}
+	t, err := time.Parse(time.RFC3339, s)
+	if err != nil {
+		return err
+	}
+	nt.Time = &t
+	return nil
+}
+
+// Value implements driver.Valuer so GORM writes NULL for nil.
+func (nt NullTime) Value() (driver.Value, error) {
+	if nt.Time == nil {
+		return nil, nil
+	}
+	return *nt.Time, nil
+}
+
+// Scan implements sql.Scanner so GORM reads NULL as nil.
+func (nt *NullTime) Scan(value interface{}) error {
+	if value == nil {
+		nt.Time = nil
+		return nil
+	}
+	t, ok := value.(time.Time)
+	if !ok {
+		nt.Time = nil
+		return nil
+	}
+	nt.Time = &t
+	return nil
+}