lib/email/send.ts: single sendEmail() over Resend's HTTP API (no SDK
dependency). Returns a retryable flag so the dispatcher distinguishes
transient (429 / 5xx / network) from permanent (config / validation)
failures. Unset RESEND_API_KEY/EMAIL_FROM is a clean non-retryable no-op
so a fresh clone still runs. Idempotency-Key wired for retry-safe sends.
Env documented in .env.example. 6 tests (fetch mocked).
