Express Idempotency Middleware

Express middleware for idempotent POST requests using Idempotency-Key header (version 1.0.2, stable release, active development). Provides pluggable stores (Memory, Redis, Postgres), in-flight concurrency control with wait or reject strategies, and safe response replay with header whitelist. TypeScript-first, ESM-only, requires Node ≥18 and Express 4 or 5. Differentiators: built-in in-flight handling via wait timeout polling, stable response fingerprinting, and separate replay header whitelist for security-aware caching of location headers without cookies or auth tokens. Designed for payment, order, and webhook idempotency scenarios.