Let \( P(a,b) \) be a point on the parabola \( y^{2}=8x \) such that the tangent at \( P \) passes through the centre of the circle \( x^{2}+y^{2}-10x-14y+65=0 \). Let \( A \) be the product of all possible values of \( a \) and \( B \) be the product of all possible values of \( b \). Then the value of \( A+B \) is equal to:
