Build Splitwise

• Users and groups; a group has members, and an expense belongs to a group (or to no group, between users)
• Add an expense with a payer, an amount, and participants, using split types equal, exact amounts and percentages; validate that exact amounts sum to the total and percentages sum to 100
• All money is stored as integer cents; an equal split of an amount that does not divide evenly distributes the remainder cents deterministically so shares always sum to the total
• balances(groupId) returns, per user, how much they are owed (positive) or owe (negative), and the net across all users is exactly zero
• simplify(groupId) returns a minimal or near-minimal list of payments (from, to, amount) that settles every balance
• Record a settlement payment between two users, which updates balances
• Expense history per group in order, supporting edit and delete with balances recomputed correctly

Only after every requirement works.

• Currency support with a fixed conversion table
• Per-pair view: how much does A owe B in total across groups
• Export the settle-up plan as a list of instructions