Mock interview: bill payments and AutoPay at Billbolt

A full 45-minute model interview: Vikram clarifies who confirms what, finds BillHub's 300 req/s as the real bottleneck, makes the pay call idempotent, contains a failing biller with per-biller breakers, makes AutoPay safe to run twice with unique runs and fencing tokens, and redoes the maths when AutoPay grows tenfold.