Fetch evidence: envelope + per-signer receipts

Returns the envelope record (including final_pdf_sha256, final_signature_base64url, final_payload_json once /finalize completes), per-signer signing receipts, and OpenTimestamps anchors. This endpoint does not return the request fingerprint: the plaintext IP / user-agent fingerprint captured at /sign time lives in the audit chain — see GET /api/envelopes/{envelope_id}/audit, inside each event's event_data_json — and is also rendered onto the in-PDF Certificate of Signatures page that ships to every recipient. Capturing it is intentional and mirrors DocuSign / Adobe Sign Certificate-of-Completion industry practice (eIDAS Art. 26, ESIGN/UETA §7); FreeSign's privacy invariant is 'no PDF bytes server-side', not 'no IPs'.