Set up default currency accounts automatically (#72)

docs/payments.md

@@ -52,22 +52,50 @@ coordinated deployment of the platform's financial services.
 
 Reuse `api.sendico` and its existing Vault-rendered token file. Do not put provider
 credentials or organization/source settings in Flutter. Provision the integration
-account's organization membership, payment-create/read, ledger-account/balance
-read and the permissions needed to resolve its stored payment methods. Verify
+account's organization membership, payment-create/read, ledger-account create/read,
+balance read and the permissions needed to resolve its stored payment methods. Verify
 those permissions in the intended environment before enabling real operations.
 
-Through Sendico's existing operator APIs, provision one dedicated unowned
-organization ledger per customer currency, with active `asset` / `operating`
-semantics, no negative balance and metadata:
+### Automatic currency accounts (#72)
+
+The backend ensures **RUB, USDT and USDC** accounts after confirmed identity intake
+during signup, before sending the activation email. Resending activation retries
+the same setup. Every authenticated wallet load also ensures the defaults, so
+existing customers receive missing accounts automatically when they next open
+their wallet. This is lazy backfill, not an eager scan of dormant customers.
+
+Setup creates dedicated unowned organization ledgers through the public
+`POST /ledger_accounts/{organization}` API, with active `asset` / `operating`
+semantics, zero initial funds, no negative balance and metadata:
 
 | Ledger metadata | Meaning |
 | --- | --- |
 | `sxchnge_subject_ref` | Confirmed independent subject reference from sign-up |
+| `external_subject_ref` | Same confirmed subject; provider-enforced organization/subject/currency uniqueness |
 | `sxchnge_payout_method_ref` | Optional prevalidated same-currency saved payout method |
 | `sxchnge_payout_label` | Safe customer-visible destination label, e.g. bank plus masked suffix |
 | `sxchnge_funding_method_ref` | Optional same-currency saved funding source supported by the platform route |
 
-These are financial-resource bindings maintained by trusted provisioning, not
+Existing customer ledgers and their balances are preserved, including manually
+provisioned ledgers with the original `sxchnge_subject_ref` tag. Only missing
+defaults are created; existing additional currencies remain visible. Repeated
+bound creates reuse the ledger and resume Sendico's durable wallet/route setup
+after an interrupted response. Frozen, duplicate or incorrectly owned bindings
+fail instead of creating a replacement. All required currency precision entries
+are checked against the provider catalog before issuing creates. Saved funding
+and payout bindings remain separate from creation of the currency accounts.
+
+Deploy [Sendico #2968](https://git.sendico.io/tech/sendico/issues/2968) first:
+Ledger (including its unique index), then Product Edge, then SXCHNGE backend and
+frontend from [task #72](https://git.sendico.io/tech/sxchnge/issues/72). The public
+ledger list must advertise `externalSubjectAccounts: true` before SXCHNGE sends
+any create requests. Older providers fail safely without creating accounts.
+The acknowledgement assumes Ledger and Edge are deployed in that order; it
+does not discover the version of a separately running Ledger. No new runtime
+configuration or credentials are introduced. Existing deployments must give the
+integration principal the ledger-create permission in addition to its reads.
+
+These are financial-resource bindings maintained by server-side setup, not
 client choices or new runtime configuration. Do not tag a pooled/integration-login
 ledger as a customer ledger. Do not place full bank/card details or secret values
 in labels or metadata. The BFF checks subject, organization, account type, role,
@@ -79,7 +107,8 @@ review action. Funding uses a quote for the provisioned stored source; it does
 not mint money or treat demo top-ups as real deposits. Additional destinations,
 recipient onboarding and other funding instructions remain separate features.
 
-An empty set of provisioned ledgers is an empty wallet. Existing organization
+If setup reports a failure, the API returns an unavailable wallet rather than an
+empty snapshot or demo balances. Retrying the app load retries setup. Existing organization
 payments and pooled balances are never imported into customer history. This
 integration exposes the durable operations initiated through this BFF, with live
 provider reconciliation; independent inbound deposits/history ingestion and