Send one OTP across a batch of envelopes (multi-document signing)

Multi-document signing (one OTP/passkey ceremony signs several PDFs). The browser creates one envelope per document, then calls this once: it verifies a per-envelope session proof for every item (carried in the body, since one request has only one set of session headers), enforces Turnstile + the per-email cooldown + rate limits ONCE, generates ONE code, and creates one OTP challenge per envelope with that shared code. A single email is sent. Each envelope is then verified individually via POST /api/envelopes/{envelope_id}/otp/verify with the one typed code (each challenge is salted by its envelope id, so the same code verifies all of them). Batch size is capped at BATCH_MAX_DOCS (default 10).