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/server/handlers.go

@@ -89,7 +89,7 @@ func (s *Server) registerRoutes() {
 		// payment over the reverse bot-link; the gateway proxies both onto these gateway-only routes.
 		s.internal.POST("/payments/telegram/precheckout", s.handleTelegramPreCheckout)
 		s.internal.POST("/payments/telegram/payment", s.handleTelegramPayment)
-		if s.robokassa.MerchantLogin != "" {
+		if s.robokassa.Configured() {
 			s.internal.POST("/payments/robokassa/result", s.handleRobokassaResult)
 		}
 	}

backend/internal/server/handlers_admin_console.go

@@ -482,7 +482,7 @@ func financeView(stmt payments.Statement) adminconsole.FinanceView {
 	for _, e := range stmt.Ledger {
 		fv.Ledger = append(fv.Ledger, adminconsole.LedgerRow{
 			Kind: e.Kind, Source: e.Source, Origin: e.Origin, ChipsDelta: e.ChipsDelta,
-			Product: e.ProductID, Order: e.OrderID, Provider: e.Provider, Snapshot: e.Snapshot,
+			Product: e.ProductID, Order: e.OrderID, Provider: e.Provider, Shop: e.Shop, Snapshot: e.Snapshot,
 			At: fmtTime(e.CreatedAt),
 		})
 	}

backend/internal/server/handlers_intake.go

@@ -12,6 +12,7 @@ import (
 	"go.uber.org/zap"
 
 	"scrabble/backend/internal/payments"
+	"scrabble/backend/internal/robokassa"
 )
 
 // Ledger/order provider tags per rail.
@@ -68,7 +69,8 @@ func (s *Server) handleWalletOrder(c *gin.Context) {
 	}
 	switch cxt.Kind {
 	case payments.SourceDirect:
-		if s.robokassa.MerchantLogin == "" {
+		shop, ok := s.robokassa.Shop(cxt.Subtype)
+		if !ok {
 			c.AbortWithStatusJSON(http.StatusNotImplemented, errorResponse{Error: errorBody{Code: "rail_unavailable", Message: "this payment method is not available"}})
 			return
 		}
@@ -89,7 +91,7 @@ func (s *Server) handleWalletOrder(c *gin.Context) {
 		}
 		c.JSON(http.StatusOK, walletOrderResponse{
 			OrderID:     res.OrderID.String(),
-			RedirectURL: s.robokassa.PaymentURL(res.OrderID, res.Amount.Major(), res.Title),
+			RedirectURL: shop.PaymentURL(res.OrderID, res.Amount.Major(), res.Title),
 			Rail:        providerRobokassa,
 		})
 	case payments.SourceVK:
@@ -136,7 +138,16 @@ func (s *Server) handleRobokassaResult(c *gin.Context) {
 	for k, val := range params {
 		v.Set(k, val)
 	}
-	orderID, outSum, ok := s.robokassa.VerifyResult(v)
+	// The per-shop Result route carries the channel as ?channel=; the legacy bare route defaults to
+	// the web shop. Each channel is verified only by its own shop's Password2 (Verifier is strict).
+	channel := c.DefaultQuery("channel", robokassa.ChannelWeb)
+	shop, ok := s.robokassa.Verifier(channel)
+	if !ok {
+		s.log.Warn("robokassa result: unknown shop channel", zap.String("channel", channel))
+		c.String(http.StatusBadRequest, "bad sign")
+		return
+	}
+	orderID, outSum, ok := shop.VerifyResult(v)
 	if !ok {
 		s.log.Warn("robokassa result: bad signature")
 		c.String(http.StatusBadRequest, "bad sign")

backend/internal/server/server.go

@@ -115,9 +115,9 @@ type Deps struct {
 	// Renderer is the image-render sidecar client for the PNG export artifact. A
 	// nil Renderer makes the PNG download answer 404 (the GCG artifact still works).
 	Renderer *render.Client
-	// Robokassa configures the direct-rail (RUB) provider; an empty MerchantLogin leaves the
-	// order and Result-callback endpoints unregistered.
-	Robokassa robokassa.Config
+	// Robokassa configures the direct-rail (RUB) provider — one merchant shop per channel; an empty
+	// set leaves the order and Result-callback endpoints unregistered.
+	Robokassa robokassa.Shops
 }
 
 // Server owns the gin engine, the underlying HTTP server and the readiness
@@ -146,7 +146,7 @@ type Server struct {
 	ads         *ads.Service
 	payments    *payments.Service
 	gamelimits  *gamelimits.Service
-	robokassa   robokassa.Config
+	robokassa   robokassa.Shops
 	notifier    notify.Publisher
 	console     *adminconsole.Renderer
 	exportKey   []byte