feat: new ValidateToken method for AuthService, based on code from the monolithic implementation of auth middleware;

feat: add detailed authentication error types;

backend/internal/errors/auth.go

@@ -29,4 +29,10 @@ var (
 	ErrInvalidCredentials = errors.New("Invalid username, password or TOTP code")
 	ErrInvalidToken = errors.New("Token is invalid or expired")
 	ErrServerError = errors.New("Internal server error")
+
+	ErrTokenExpired = errors.New("Token is expired")
+	ErrTokenInvalid = errors.New("Token is invalid")
+	ErrWrongTokenType = errors.New("Invalid token type")
+	ErrSessionNotFound = errors.New("Could not find session in database")
+	ErrSessionTerminated = errors.New("Session is terminated")
 )