Stir together yogurt, mint, garlic, and salt in a small bowl and chill. Whisk together lemon juice, sugar, salt, pepper, and oil in a large bowl and stir in zucchini slices. Marinate at room temperature while making meatballs. Cover bread with water in a bowl and soak 10 minutes. Squeeze handfuls of bread to remove as much excess water as possible, then transfer to a bowl. Pulse onion and herbs in a food processor until finely chopped, then add to bread along with lamb, salt, spices, and pine nuts. Mix with your hands until well blended. Form lamb mixture into 36 balls (1 scant tablespoon each). Prepare grill for cooking over medium-hot charcoal (moderate heat for gas; see cooks` note below). Thread 6 meatballs 1/4 inch apart onto each of 6 skewers. Thread zucchini lengthwise onto remaining 6 skewers (5 slices per skewer), so cut sides are on the grill, leaving 1/4 inch between slices. Grill zucchini and lamb on oiled grill rack, turning over once, until golden and just cooked through, 4 to 6 minutes. Serve warm, with yogurt sauce.