Rate limit headers

Every successful response carries:

• X-RateLimit-Limit — your tier's daily cap
• X-RateLimit-Remaining — what's left today
• X-RateLimit-Reset — Unix timestamp of the next daily rollover (midnight Europe/Copenhagen)
• X-RateLimit-Limit-Month / -Remaining-Month / -Reset-Month — same, for tiers with a monthly cap

A 429 additionally carries Retry-After (seconds until the window resets), per RFC 6585. X-RateLimit-Remaining: 0 is set explicitly so an SDK does not have to infer the exhaustion.