Moroccan Pork Kebabs

Name: Moroccan Pork Kebabs
Ingredients: 1/4 cup orange juice; 1 tablespoon tomato paste; 1 clove garlic, finely chopped; 1 tablespoon ground cumin; 1/8 teaspoon ground cinnamon; 4 tablespoons olive oil; 1 1/2 teaspoons kosher salt; 3/4 teaspoon black pepper; 1 1/2 pounds boneless pork loin, cut into 1 1/2-inch chunks; 1 small eggplant, unpeeled, cut into 1-inch chunks; 1 small red onion, cut into 8 wedges; pita bread or flat bread, for serving; 1 half-pint container store-bought tzatziki (cucumber-yogurt sauce; optional); 1/2 small cucumber, thinly sliced; 2 tablespoons chopped fresh mint; wooden skewers, soaked in water for 30 minutes