Peel the shrimp. Put the shells in a small saucepan with the tomato, water, clam juice, chile flakes, paprika, and salt. Bring to a boil, reduce to a simmer, cover, and cook for 10 minutes.
Meanwhile, heat a 10-inch skillet with the oil on medium-low heat. Add the onion and cook, stirring frequently, until softened, about 6 minutes. Add the garlic and cook until softened, 2 to 3 minutes.
Remove the shrimp-shell broth from the heat and strain through a sieve into a 4-cup measure. You should have 2-3/4 cups liquid; if there is less, add water to compensate.
Increase the heat under the skillet to medium high. Stir in the rice, and cook, stirring frequently, until it becomes somewhat translucent, about 1 minute. Add the strained broth, spread the rice into an even layer, and bring to a boil. Reduce the heat to medium and simmer vigorously for 5 minutes. Arrange the shrimp on top of the rice and simmer until the rice and liquid are at the same level, about another 5 minutes. Reduce the heat to medium low and simmer gently until the rice is just tender and the liquid is absorbed, about another 10 minutes. (It’s all right if the rice starts to stick to the bottom of the pan—it can be brown— but don’t let it burn.) Remove from the heat, cover, and let rest for 10 minutes. Sprinkle with the parsley, if using, and serve with the lemon wedges.