feat/1019-signed-evidence-bundles-transparency-log

[nasarajoe]

Summary

Implements signed, portable evidence bundles and an append-only transparency
log so a verifier can prove exactly which raw observations, code/config
versions, decoder versions, query parameters and chain-finality metadata
produced a report or export — without trusting the application database.

Closes #1019

What's included

Pure verification core — backend/src/services/transparencyLog/

No DB or network imports, so the exact same code backs the offline verifier.

• canonical.ts — deterministic JSON (RFC 8785 subset)
• merkle.ts — RFC 6962 Merkle tree: root, inclusion proofs, consistency
  proofs, and tree-free verification (ported from transparency-dev/merkle)
• ed25519.ts — Ed25519 over raw 32-byte hex keys
• evidenceBundle.ts — canonical core builder, evidence-root hashing,
  partial disclosure (discloseSubset), and verifyEvidenceBundle() which
  recomputes every commitment and checks the signature + transparency proof

Database service — backend/src/services/evidenceBundle.service.ts

• Rotating Ed25519 signer keys; registration / rotation / revocation each
  appended to the log so key lifecycle is independently auditable
• Append-only log serialized with pg_advisory_xact_lock, publishing a
  signed tree head for every tree size (signed by a separate log key so
  tree-head trust and authorship trust are distinct)
• Assembles a self-contained bundle document: canonical core + disclosed
  sections (with salts) + inclusion proof + signed tree head + signer
  key-registration proof
• Inclusion and consistency proof serving

Schema — 20260829100000_signed_evidence_bundles.ts

evidence_bundle_signing_keys, transparency_log_entries,
transparency_log_checkpoints, evidence_bundles.

REST API — /api/v1/evidence/*

Method	Path	Purpose
POST	/bundles	create a signed bundle for a report/export
GET	/bundles / /bundles/:id	list / fetch full bundle document
GET	/bundles/:id/disclose	partial-disclosure view (?sections=&outputs=)
GET	/bundles/:id/verify	server-side offline verification report
POST	/bundles/verify	stateless verification of a supplied document
GET	/log / /log/checkpoint	log entries / latest signed tree head
GET	/log/proof/inclusion / /log/proof/consistency	proofs
GET	/log/keys, POST /log/keys/rotate, POST /log/keys/:keyId/revoke	signer key lifecycle

Standalone offline verifier — backend/scripts/verify-evidence-bundle.mjs

Node stdlib only. Prints a PASS/FAIL line per check; exit 0 = valid, 1 = invalid.

curl -s "$API/api/v1/evidence/bundles/eb_abc123" | node backend/scripts/verify-evidence-bundle.mjs -