ValidateAndFill now checks the DB query result and returns sentinel errors
(ErrDatabase, ErrInvalidCredentials, ErrUserEmptyCredentials) instead of
hardcoded Chinese strings. The controller maps each sentinel to the
appropriate i18n message, so users see "please contact admin" on DB errors
instead of a misleading "wrong password" message. Non-DB errors still
return a unified vague response to avoid leaking user existence.
