Banking System (Levels 1–4)

Design a banking system that supports the operations: create_account, deposit, pay, top_activity (top-N accounts by outgoing volume), transfer (initiated; recipient must accept within 24h), accept_transfer, merge_accounts, and get_balance(time_at) (balance at a historical timestamp). Methods are called with monotonic timestamps.

Example: deposit, then pay, then get_balance at the deposit timestamp returns the post-deposit balance, while at a later timestamp returns the post-pay balance.