refactor: updated swagger;

feat: helper function in errors for checking postgres error types;
feat: sql query method for finding users by their email;
feat: registration begin/complete with checking existing username/email;
refactor: error handling in controller

backend/internal/errors/auth.go

@@ -24,6 +24,7 @@ import (
 var (
 	ErrUnauthorized = errors.New("User is not authorized")
 	ErrUsernameTaken = errors.New("Provided username is already in use")
+	ErrEmailTaken = errors.New("Provided email is already in use")
 	ErrUserNotFound = errors.New("User was not found")
 	ErrInvalidCredentials = errors.New("Invalid username, password or TOTP code")
 	ErrInvalidToken = errors.New("Token is invalid or expired")