refactor: use dependency injection in backend

backend/internal/common/errors.go

@@ -0,0 +1,18 @@
+package common
+
+import "errors"
+
+var (
+	ErrUsernameTaken                = errors.New("username is already taken")
+	ErrEmailTaken                   = errors.New("email is already taken")
+	ErrSetupAlreadyCompleted        = errors.New("setup already completed")
+	ErrInvalidBody                  = errors.New("invalid request body")
+	ErrTokenInvalidOrExpired        = errors.New("token is invalid or expired")
+	ErrOidcMissingAuthorization     = errors.New("missing authorization")
+	ErrOidcGrantTypeNotSupported    = errors.New("grant type not supported")
+	ErrOidcMissingClientCredentials = errors.New("client id or secret not provided")
+	ErrOidcClientSecretInvalid      = errors.New("invalid client secret")
+	ErrOidcInvalidAuthorizationCode = errors.New("invalid authorization code")
+	ErrFileTypeNotSupported         = errors.New("file type not supported")
+	ErrInvalidCredentials           = errors.New("no user found with provided credentials")
+)