refactor: cleanup unused code and simplify internal packages

This commit performs a significant cleanup of the codebase by removing unused functions, methods, and entire files across various internal modules. This reduces technical debt and simplifies the project structure.

Key changes include:
- **cache**: Removed unused cache key generators and metrics snapshots.
- **dto**: Removed redundant converter functions and segment creation helpers.
- **middleware**: Deleted the unused `logger.go` middleware and simplified `ratelimit.go` and `casbin.go`.
- **model**: Removed unused ID helpers, database closing functions, and batch decryption logic.
- **pkg**: Cleaned up unused utility functions in `circuitbreaker`, `crypto`, `cursor`, `hook`, and `utils`.
- **service**: Deleted `account_cleanup_service.go` and removed unused helper functions in `log_cleanup_service.go` and `sensitive_service.go`.
- **repository**: Removed unused private loading methods in `comment_repo.go`.

internal/errors/app_errors.go

@@ -151,16 +151,6 @@ func Wrap(err error, appErr *AppError) *AppError {
 	}
 }
 
-// New 创建新错误
-func New(code, message string) *AppError {
-	return &AppError{Code: code, Message: message}
-}
-
-// Is 判断是否为特定错误
-func Is(err, target error) bool {
-	return errors.Is(err, target)
-}
-
 // As 将错误转换为 AppError
 func As(err error) (*AppError, bool) {
 	var appErr *AppError