refactor(errs): migrate from gperr.Error to standard Go error interface

This is a large-scale refactoring across the codebase that replaces the custom
`gperr.Error` type with Go's standard `error` interface. The changes include:

- Replacing `gperr.Error` return types with `error` in function signatures
- Using `errors.New()` and `fmt.Errorf()` instead of `gperr.New()` and `gperr.Errorf()`
- Using `%w` format verb for error wrapping instead of `.With()` method
- Replacing `gperr.Subject()` calls with `gperr.PrependSubject()`
- Converting error logging from `gperr.Log*()` functions to zerolog's `.Err().Msg()` pattern
- Update NewLogger to handle multiline error message
- Updating `goutils` submodule to latest commit

This refactoring aligns with Go idioms and removes the dependency on
custom error handling abstractions in favor of standard library patterns.

internal/api/v1/file/validate.go

@@ -51,7 +51,7 @@ func Validate(c *gin.Context) {
 	c.JSON(http.StatusOK, apitypes.Success("file validated"))
 }
 
-func validateFile(fileType FileType, content []byte) gperr.Error {
+func validateFile(fileType FileType, content []byte) error {
 	switch fileType {
 	case FileTypeConfig:
 		return config.Validate(content)