Build a small `rate_limited(user_id, requests_per_minute, current_rpm)` that returns the HTTP status code you'd send: 1. `current_rpm < requests_per_minute` → `200` (allowed). 2. `current_rpm >= requests_per_minute` → `429` (Too Many Requests). 3. `user_id` starting with `"admin_"` is exempt — always `200`. 4. `requests_per_minute <= 0` is a misconfiguration — return `503` (Service Unavailable) so the caller sees a config bug, not a silent block.