feat(payments): direct-rail order + intake handlers, config and reaper

Wire the Robokassa direct rail into the backend transport. POST
/api/v1/user/wallet/order (walletGate + a D36 confirmed-email gate for the
direct rail) opens a pending order and returns the signed Robokassa payment
URL. The internal, gateway-only /payments/robokassa/result endpoint verifies
the Result signature, credits the matched order exactly once via Fund (honoured
even if expired), records a succeeded payment event, and answers Robokassa's
"OK<InvId>". Add the Robokassa env config, an account HasConfirmedEmail check
(D36), the payment_events writer, and a periodic pending-order reaper. The
routes register only when a Robokassa merchant login is configured.

backend/internal/server/server.go

@@ -31,6 +31,7 @@ import (
 	"scrabble/backend/internal/payments"
 	"scrabble/backend/internal/ratewatch"
 	"scrabble/backend/internal/render"
+	"scrabble/backend/internal/robokassa"
 	"scrabble/backend/internal/session"
 	"scrabble/backend/internal/social"
 	"scrabble/backend/internal/telemetry"
@@ -109,6 +110,9 @@ 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
 }
 
 // Server owns the gin engine, the underlying HTTP server and the readiness
@@ -136,6 +140,7 @@ type Server struct {
 	banview     *banview.View
 	ads         *ads.Service
 	payments    *payments.Service
+	robokassa   robokassa.Config
 	notifier    notify.Publisher
 	console     *adminconsole.Renderer
 	exportKey   []byte
@@ -189,6 +194,7 @@ func New(addr string, deps Deps) *Server {
 		banview:       deps.BanView,
 		ads:           deps.Ads,
 		payments:      deps.Payments,
+		robokassa:     deps.Robokassa,
 		notifier:      notifier,
 		renderer:      deps.Renderer,
 		http:          &http.Server{Addr: addr, Handler: engine},