Send an envelope

Transitions the envelope to in_progress in one transaction: validates the stored document (presence + pdf|docx content type; upload-mode envelopes get their contentHash/sizeBytes computed from the stored bytes), requires ≥1 participant, appends envelope.sent and participant.link_issued for the first signer (version bumps twice). Emits the atlas/envelope.sent notification event (identifiers only). Supports the Idempotency-Key header: a replayed completed key returns the envelope's current graph with the original status code and Idempotency-Replayed: true (never re-executing); an in-flight duplicate is rejected 409; reusing a key with a different request is rejected 422.