feat: Add user notification settings with quota warning and multiple notification methods

- Implement user notification settings with email and webhook options
- Add new user settings for quota warning threshold and notification preferences
- Create backend API and database support for user notification configuration
- Enhance frontend personal settings with notification configuration UI
- Support custom notification email and webhook URL
- Add service layer for sending user notifications

dto/notify.go

@@ -0,0 +1,24 @@
+package dto
+
+type Notify struct {
+	Type    string        `json:"type"`
+	Title   string        `json:"title"`
+	Content string        `json:"content"`
+	Values  []interface{} `json:"values"`
+}
+
+const ContentValueParam = "{{value}}"
+
+const (
+	NotifyTypeQuotaExceed   = "quota_exceed"
+	NotifyTypeChannelUpdate = "channel_update"
+)
+
+func NewNotify(t string, title string, content string, values []interface{}) Notify {
+	return Notify{
+		Type:    t,
+		Title:   title,
+		Content: content,
+		Values:  values,
+	}
+}