feat: Add public user information retrieval endpoint

- Introduced a new endpoint to fetch public user information without authentication.
- Implemented UserToPublicUserInfo function to format user data for the response.
- Updated UserService interface and user service implementation to support fetching users by username.
- Enhanced user handler to validate input parameters and check user status before responding.

internal/types/common.go

@@ -110,6 +110,18 @@ type UserInfo struct {
 	UpdatedAt   time.Time  `json:"updated_at" example:"2025-10-01T10:00:00Z"`
 }
 
+// PublicUserInfo 用户公开信息
+// @Description 用户公开信息（不包含敏感信息如邮箱）
+type PublicUserInfo struct {
+	ID        int64     `json:"id" example:"1"`
+	Username  string    `json:"username" example:"testuser"`
+	Avatar    string    `json:"avatar" example:"https://example.com/avatar.png"`
+	Points    int       `json:"points" example:"100"`
+	Role      string    `json:"role" example:"user"`
+	Status    int16     `json:"status" example:"1"`
+	CreatedAt time.Time `json:"created_at" example:"2025-10-01T10:00:00Z"`
+}
+
 // TextureType 材质类型
 type TextureType string