Solution:

The signs and the alternation between sines and cosines suggest powers of $i:$ indeed $$Q + iP = 1 + \frac{1}{2}ie^{i\theta} + \frac{1}{4}i^2e^{2i\theta} + \frac{1}{8}i^3e^{3i\theta} + \cdots = \frac{1}{1 - \frac{ie^{i\theta}}{2}} = \frac{2}{2 - ie^{i\theta}}.$$ Since $2 - ie^{i\theta} = (2 + \sin\theta) - i\cos\theta,$ multiplying by the conjugate gives $$Q + iP = \frac{2(2 + \sin\theta) + 2i\cos\theta}{5 + 4\sin\theta},$$ so $\frac{P}{Q} = \frac{\cos\theta}{2 + \sin\theta}.$

Setting $\frac{\cos\theta}{2 + \sin\theta} = \frac{2\sqrt{2}}{7}$ and squaring, $49(1 - \sin^2\theta) = 8(2 + \sin\theta)^2,$ which rearranges to $$57\sin^2\theta + 32\sin\theta - 17 = (3\sin\theta - 1)(19\sin\theta + 17) = 0.$$

Since $\pi \le \theta \lt 2\pi$ forces $\sin\theta \le 0,$ we get $\sin\theta = -\frac{17}{19}$ (and then $\cos\theta = \frac{6\sqrt{2}}{19} \gt 0,$ consistent with the positive ratio). Thus $m + n = 17 + 19 = 36.$