Audit trail + public verification

Every action on an envelope writes a row to the event log. Each row's SHA-256 hash includes the previous event's hash, the sequence number, the event type, the timestamp, and a canonical JSON of the payload. Altering any earlier event breaks the chain — visible on the public verify page and detectable by anyone.

When the last recipient signs, we stamp signatures onto the PDF, append a Certificate of Completion page (or pages, for long event logs), re-hash the result, and store it. The certificate lists the envelope ID, both SHA-256 hashes, every recipient with timestamps and method (typed/drawn/uploaded), the full event log, and a link to the verify page.

Anyone with the envelope ID can visit /verify and paste it in. The page shows the original + signed SHA-256, the audit chain head hash, how many signers signed, and the event log (names and file names are shown only to the sender’s team). If the chain is intact you get a green banner; if it's been tampered with, a red banner names the broken event.