Separate second-factor verification and delivery from auth (#40)

docs/implementation.md

@@ -536,3 +536,47 @@ frontend API URL/public client before enabling real auth.
 
 
 Delivered in [PR #39](https://git.sendico.io/tech/sxchnge/pulls/39). Merge order: #35 → #37 → #39. The auth branch incorporates notification completion commit `4b14649`; only the delivery-log append conflicted, and both task records are preserved.
+
+## Dedicated second-factor service (#40)
+
+Task [#40](https://git.sendico.io/tech/sxchnge/issues/40), branch `twofactor-40`,
+based on main after merged PR #39. Primary module: twofactor; secondary modules:
+auth, verification purpose definitions, notification templates and Flutter auth.
+
+`product/twofactor` now owns challenge issuance/delivery, verified destinations,
+preferences, attempt/cooldown limits and single-use proofs. It reuses the
+Sendico-derived verification store at the existing pinned revision. No new
+Sendico handler or private messaging code was copied. Auth owns credentials,
+pending JWTs and session creation; signup activation remains in `auth/flow`.
+The service's `Complete` transaction commits code verification, proof consumption
+and the owner's session write together. Review fixed rejected-attempt rollback,
+ensured failed session writes preserve retryability, retained activation replay
+key compatibility and removed the unused verification-store injection from auth
+HTTP. A profile write serializes consumption against preference changes.
+
+Authenticated APIs enroll additional verified emails and change the preferred
+factor only after verifying the current factor. Unsupported channels fail closed.
+Flutter's shared DTO/mapper/service adapter sends the server challenge ID with
+the pending JWT, preserves resend keys after failure and rejects unknown channels.
+No widgets or provider/analytics boundaries changed. Existing login journey
+measurement and error/recovery UI remain in use; settings UI integration is not
+part of this separation.
+
+Validation passed: `make test` (all Go race suites and `make test-frontend`),
+`make lint-backend`, `flutter analyze` in `frontend/packages/sxchnge_auth`, and
+Dart format verification for all four changed Dart files. New real Mongo tests
+cover single-use proofs, customer/purpose/context isolation, delivery retries,
+committed failed guesses, expiry, inactive accounts, operation rollback,
+concurrent completion, verified enrollment, preference selection and old-proof
+invalidation. The updated signup/login test covers the full auth boundary;
+transport and architecture tests cover scope injection and domain dependencies.
+Frontend tests cover challenge binding, resend recovery and unsupported channels.
+No live email or optional security scan was run.
+
+See [ownership, Sendico provenance, HTTP contract and rollout](architecture/two-factor.md).
+Deploy backend and client together, load `twofactor.code` revision 1, and restart
+previously pending logins after rollout. Email is the implemented channel.
+Existing settings screens still use preview adapters; SMS/push/TOTP/passkeys,
+factor removal/recovery, payout integration and a separate process are not
+implemented. Successful-commit response loss still requires login restart;
+notification queue acceptance is not inbox delivery.