feat(payments): per-channel Robokassa shops on the direct rail

Split the single Robokassa direct rail into one merchant shop per channel
(web / android; ios later), chosen by the trusted X-Platform subtype, while
every shop still credits the one `direct` wallet — merchant-account separation
for accounting and receipts, not a new wallet.

- config: a channel-keyed shops registry (web seeds from the legacy vars or
  _WEB_*, android from _ANDROID_*); an empty shop leaves the rail dormant;
  per-shop validation.
- intake: the order picks its shop by subtype (unknown falls back to web); the
  per-shop Result callback is verified by that shop's own Password2 at
  /pay/robokassa/result/<channel> (the gateway extracts the channel; Caddy's
  /pay/* glob already forwards it — no Caddyfile change).
- persistence: an additive `shop` column on the order (migration 00015),
  recorded from the payment context, surfaced per entry in the admin report.
- standalone apps sign in by email only, so a direct purchase keeps its email
  anchor.
- docs: PAYMENTS (+ru) topology, deploy env vars + compose mapping, the
  decisions log (D41 revised for the ИП / 54-ФЗ move; D42-D44), the plan.

Contour-safe: dormant until shops are configured; the migration is additive
(no wipe); no client wire change. Fiscalization (Receipt/Email) and the gateway
`direct/android` subtype follow when the ИП / RuStore are live.

backend/internal/payments/store_statement.go

@@ -74,9 +74,39 @@ func (s *Store) accountStatement(ctx context.Context, accountID uuid.UUID) (Stat
 			CreatedAt:         r.CreatedAt,
 		})
 	}
+
+	// Annotate direct-rail entries with the merchant shop (channel) their order was issued through
+	// (E10/D44), keyed by the ledger's order id. Non-direct / order-less entries stay "".
+	shops, err := s.orderShops(ctx, accountID)
+	if err != nil {
+		return Statement{}, err
+	}
+	for i := range out.Ledger {
+		out.Ledger[i].Shop = shops[out.Ledger[i].OrderID]
+	}
 	return out, nil
 }
 
+// orderShops maps an account's order ids (as strings) to the merchant shop (channel) each direct
+// order was issued through, for annotating the ledger report (E10/D44). Orders with an empty shop
+// (non-direct or pre-split) are omitted, so a lookup miss yields "".
+func (s *Store) orderShops(ctx context.Context, accountID uuid.UUID) (map[string]string, error) {
+	var rows []model.Orders
+	if err := postgres.SELECT(table.Orders.OrderID, table.Orders.Shop).
+		FROM(table.Orders).
+		WHERE(table.Orders.AccountID.EQ(postgres.UUID(accountID))).
+		QueryContext(ctx, s.db, &rows); err != nil {
+		return nil, fmt.Errorf("payments: load order shops %s: %w", accountID, err)
+	}
+	m := make(map[string]string, len(rows))
+	for _, r := range rows {
+		if r.Shop != "" {
+			m[r.OrderID.String()] = r.Shop
+		}
+	}
+	return m, nil
+}
+
 // allLedger reads the entire append-only ledger (all accounts, newest first) for the admin export.
 func (s *Store) allLedger(ctx context.Context) ([]LedgerExportRow, error) {
 	var rows []model.Ledger