feat: 实现用户通知系统，包含通知模型、数据操作、业务逻辑及相关API接口。

internal/models/notification.go

@@ -0,0 +1,34 @@
+package models
+
+import (
+	"time"
+)
+
+// NotificationType represents the type of notification
+type NotificationType string
+
+const (
+	NotificationTypeSystem NotificationType = "system"
+	NotificationTypeAlert  NotificationType = "alert"
+	NotificationTypeInfo   NotificationType = "info"
+)
+
+// Notification represents a system notification for a user
+type Notification struct {
+	BaseModel
+	UserID  uint             `gorm:"not null;index" json:"user_id"`
+	Title   string           `gorm:"size:200;not null" json:"title"`
+	Content string           `gorm:"type:text;not null" json:"content"`
+	Type    NotificationType `gorm:"size:20;not null;default:'system'" json:"type"`
+	IsRead  bool             `gorm:"default:false" json:"is_read"`
+	Link    string           `gorm:"size:255" json:"link,omitempty"` // Optional link to redirect
+	ReadAt  *time.Time       `json:"read_at,omitempty"`
+
+	// Relationships
+	User User `gorm:"foreignKey:UserID" json:"-"`
+}
+
+// TableName specifies the table name for Notification
+func (Notification) TableName() string {
+	return "notifications"
+}