Validate email deliverability

Batch deliverability check (up to 100 addresses) — run BEFORE importing or sending. Each address is classified valid (safe), risky (deliverable but flagged — e.g. role account, disposable, catch-all), or invalid (undeliverable / do-not-send), with a machine-readable reason and a didYouMean typo correction when applicable. Cost: 2 credits PER ADDRESS (X-Credit-Cost = 2 × the number of addresses), charged ONLY on success — if the check cannot complete for the whole batch, the call returns a retryable 503 and is NOT billed.