For the iced tea: Pour 4 cups boiling water over teabags in a heatproof pitcher and allow to brew, 3 to 5 minutes. Remove teabags and add sweetener to taste. Stir in 6 cups ice cubes until melted. Keep refrigerated until ready to use. Makes two quarts.
For the raita: Whisk the yogurt, lime zest, cayenne and a pinch of salt together in a medium bowl until smooth. Whisk in enough water to make the yogurt barely pourable. Stir the cucumber and mint into the yogurt. Set aside.
For the burgers: Preheat a grill for cooking over medium-high heat. Cut the lettuce into thin strips and coarsely chop the cilantro leaves. Toss together in a small bowl and refrigerate until ready to serve.
Heat the oil in a medium skillet over medium-high heat. Add the onions and cook, stirring occasionally, until tender and browned, about 4 minutes. Add the bell peppers, zucchini and 1 teaspoon of salt and cook, stirring occasionally, until tender and beginning to brown, 3 to 5 minutes. Add the curry powder, garam masala, and ground ginger and cook, stirring, 1 minute more. Transfer to a plate and refrigerate for about 5 minutes to cool.
While the vegetables cool, stir together the chickpeas and pumpkin in a medium bowl. Add the cooled vegetables, breadcrumbs, 1 teaspoon salt and some pepper and stir with a rubber spatula until combined.
Divide the mixture into 4 equal portions and form each portion into a patty. Rub oil on the grill grates and place the patties on the grill. Grill 6 minutes on each side, then transfer to a plate.
Place a dollop of raita on the bottoms of the buns, then top with a patty, another dollop of the raita, some of the lettuce and cilantro mix and cover with the bun tops. Serve with lime wedges and iced tea.