Combine the chili powder and lime juice in a medium bowl. Add the turkey and turn to coat; let marinate 10 minutes. Bring a medium saucepan of water to a boil. Add the couscous and cook until tender, 8 to 10 minutes; drain. Meanwhile, heat 1 tablespoon olive oil in a large cast-iron skillet over medium-high heat. Add the corn, black-eyed peas and celery and cook, stirring occasionally, until they begin to soften, about 3 minutes. Add the tomatoes, scallions, parsley, 1/2 teaspoon salt, and pepper to taste and cook until warmed through, about 2 more minutes; transfer to a large bowl. Add the couscous to the bowl along with 1 tablespoon olive oil, and salt and pepper to taste; toss. Wipe out the skillet; add the remaining 1 tablespoon olive oil and heat over medium-high heat. Add the turkey and cook until browned, about 3 minutes per side; add salt and pepper. Serve with the couscous.