POST /api/auth/register
POST /api/auth/login
GET /api/auth/me
POST /api/groups
GET /api/groups
GET /api/groups/:groupId
POST /api/groups/:groupId/members
DELETE /api/groups/:groupId/members/:userId
DELETE /api/groups/:groupId/leave
POST /api/expenses
GET /api/expenses/group/:groupId
GET /api/expenses/:expenseId
DELETE /api/expenses/:expenseId
POST /api/settlements
GET /api/settlements/group/:groupId
PATCH /api/settlements/:settlementId/complete
GET /api/balances/group/:groupId
GET /api/balances/me