Financial account ledger with acceptance, limits and settlement

Implement process_ledger(commands), which handles commands in order: set up accounts, request deposits and query balances. Return the query answers as one comma-separated string.

Each deposit method has its own settlement delay, cut-off time and limit, so a deposit goes through separate stages: when the request is accepted, when it counts against the limit, and when the money settles into the balance. The task grows over four parts; keep those stages apart so each new rule fits in.