[Elective 4-4: Coordinate Systems and Parametric Equations]
Given $P(2,1)$ and line $l : \left\{ \begin{array}{l} x = 2 + t\cos\alpha \\ y = 1 + t\sin\alpha \end{array} \right.$ ($t$ is a parameter). Line $l$ intersects the positive $x$-axis and positive $y$-axis at points $A , B$ respectively, with $|PA| \cdot |PB| = 4$ .
(1) Find the value of $\alpha$ ;
(2) With the origin as the pole and the positive $x$-axis as the polar axis, find the polar equation of line $l$ .