Connect customer quotations and payment execution to Sendico (#50)

docs/implementation.md

@@ -927,3 +927,40 @@ routes proof cases to Identity proof review while blocking generic login linking
 and assignment. Apply #2886 before #2887 and deploy the combined Sendico result
 before this SXCHNGE flow. The public proof/limits contract is unchanged; this
 SXCHNGE follow-up changes documentation only and requires consistency/diff checks.
+
+## Sendico quotations and payment execution — task #50
+
+[Task #50](https://git.sendico.io/tech/sxchnge/issues/50), branch `payments-50`,
+connects backend wallet mode through the authenticated customer BFF to Sendico's
+public account, quotation, by-quote execution and payment-status APIs. It is
+stacked on `identity-verification-48` / PR #49; merge #47 → #49 first.
+
+The separate public-provider prerequisite is
+[Sendico PR #2897](https://git.sendico.io/tech/sendico/pulls/2897), task #2896.
+It validates independent customer context and selects the reviewed ACCOUNT limits
+resource rather than the integration login or an unassigned CUSTOMER policy.
+Deploy its Product Edge and Quotation/Orchestrator consumers together before
+SXCHNGE. The BFF requires an exact customer-context acknowledgement from quotes.
+
+The contribution adds durable customer/namespace/subject-bound quotation ownership,
+transactional payment-factor authorization, unique quote/execution keys, stable
+provider replay, payment-reference reconciliation and authoritative status polling.
+Live UI uses provisioned destinations, accurate quote expiry, explicit missing
+method/loading/error/pending states and long labels at narrow widths. It clears
+account state on customer changes. DTO/mapper/service/model/provider boundaries,
+shared database wrappers, existing credentials and explicit demo mode are retained.
+
+Validation passed: `make test`, `make lint-backend`, `make test-frontend` after the
+final UI review, changed-Dart format verification, and app/wallet `flutter analyze`.
+The final frontend run passed 21 wallet and 81 app tests, plus the unchanged shared
+package suites. Tests include synthetic public HTTP, Mongo transaction rollback,
+unique-key/customer isolation, lost-response reconciliation, factor retry and
+session replacement. Sendico's full backend test/lint targets also passed.
+
+[Payment contracts, provisioning, UX measurement and limitations](payments.md)
+document rollout. Dedicated customer ledger/method bindings and platform access
+must be provisioned through existing operator APIs; this work does not run live
+payments or deploy either repository. Generic ambiguous rejections remain pending
+for operational reconciliation. Cross-session analytics retention, additional
+funding/recipient onboarding, backend locale saving and external history ingestion
+remain separate capabilities. Unrelated notifications runtime changes are excluded.