For what values of $\theta$ on the polar curve $r=\theta$, with $0\le\theta\le2\pi$, are the tangent lines horizontal? Vertical?

Tangent lines are horizontal for $\theta =$
(If there is more than one value, give them separated by commas.)

Tangent lines are vertical for $\theta =$
(If there is more than one value, give them separated by commas.)