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/service/user_service.go

@@ -199,6 +199,14 @@ func (s *userService) GetByEmail(ctx context.Context, email string) (*model.User
 	}, s.cache.Policy.UserEmailTTL)
 }
 
+func (s *userService) GetByUsername(ctx context.Context, username string) (*model.User, error) {
+	// 使用 Cached 装饰器自动处理缓存
+	cacheKey := s.cacheKeys.UserByUsername(username)
+	return database.Cached(ctx, s.cache, cacheKey, func() (*model.User, error) {
+		return s.userRepo.FindByUsername(ctx, username)
+	}, s.cache.Policy.UserTTL)
+}
+
 func (s *userService) UpdateInfo(ctx context.Context, user *model.User) error {
 	err := s.userRepo.Update(ctx, user)
 	if err != nil {