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_intake.go

@@ -151,6 +151,7 @@ type newOrder struct {
 	amount    Money
 	origin    Source
 	provider  string
+	shop      string
 }
 
 // createOrder inserts a pending order.
@@ -159,12 +160,12 @@ func (s *Store) createOrder(ctx context.Context, o newOrder, now time.Time) erro
 		table.Orders.OrderID, table.Orders.AccountID, table.Orders.Platform,
 		table.Orders.ProductID, table.Orders.ExpectedAmount, table.Orders.Currency,
 		table.Orders.Origin, table.Orders.Status, table.Orders.Provider,
-		table.Orders.CreatedAt, table.Orders.UpdatedAt,
+		table.Orders.CreatedAt, table.Orders.UpdatedAt, table.Orders.Shop,
 	).VALUES(
 		o.orderID, o.accountID, o.platform,
 		o.productID, o.amount.Minor(), string(o.amount.Currency()),
 		string(o.origin), "pending", o.provider,
-		now, now,
+		now, now, o.shop,
 	)
 	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
 		return fmt.Errorf("payments: create order: %w", err)