add auth module

backend/internal/auth/errors.go

@@ -0,0 +1,12 @@
+package auth
+
+import "errors"
+
+var (
+	ErrInvalidCredentials = errors.New("invalid credentials")
+	ErrUserExists         = errors.New("user already exists")
+	ErrUserNotFound       = errors.New("user not found")
+	ErrPasswordMismatch   = errors.New("passwords do not match")
+	ErrCredentialsEmpty   = errors.New("username and password cannot be empty")
+	ErrPasswordTooShort   = errors.New("password must be at least 6 characters long")
+)