API Rate Limiter

Design and implement API rate limiting for any backend service. Includes production-ready middleware for Express, Fastify, FastAPI, Flask, and Django with four algorithm options (token bucket, sliding window log, sliding window counter, fixed window). Features Redis-backed distributed rate limiting for multi-instance deployments, tiered limits for free/paid plans, per-IP/per-user/per-API-key strategies, proper HTTP response headers (X-RateLimit-Limit, Remaining, Reset, Retry-After), allowlist bypass, per-endpoint configuration, graceful degradation patterns, and monitoring integration. Every API needs rate limiting — this skill gets it right the first time.