feat(payments): send no fiscal receipt; keep the code switchable

The merchant accepts payments as a sole proprietor on НПД, which is outside
54-ФЗ: there is no online cash register, YooKassa does not serve receipts for
that regime at all (checked with their support), and each operation is reported
by the merchant to «Мой налог», which issues the чек. So no `receipt` is sent
with a payment or a refund.

This also removes a failure class rather than just code: a malformed receipt
was an API error at payment creation, which broke the purchase outright.

The fiscal code is kept dormant rather than deleted. All of it now sits behind
one switch, `BACKEND_YOOKASSA_VAT_CODE`: unset — the default — builds and sends
nothing; a 54-ФЗ rate code turns «Чеки от ЮKassa» back on unchanged. The return
is foreseeable, which is why the switch exists: НПД carries an annual income
ceiling, and losing the regime puts 54-ФЗ back in force, at which point this is
a deploy-variable edit instead of writing the integration again.

The D36 email anchor still gates a direct purchase. It had two justifications —
a recovery anchor and the receipt address — and only the second is gone; without
an email a paying customer who loses the account loses the chips with it. What
was missing is that the rule was enforced but never communicated: the wallet
showed the packs to a player signed in through VK or Telegram in a browser, and
tapping Buy produced a bare "something went wrong". It now says "add an email in
your profile" in the buy tab instead, linking to the profile; spending
already-earned chips is untouched. The predicate is a pure function so it is
covered by the node-env unit tests rather than needing a browser.

Tests: the receipt-off default is pinned by an integration test asserting a
purchase carries no receipt, and the dormant path by one that switches a VAT
code on and checks the receipt reappears with the right fiscal attributes;
plus unit coverage for the enable predicate and the wallet's email rule.

A note for whoever runs the numbers next: the shared (svelte + i18n) chunk is
now 40 bytes under its 31 KB gzip budget.

Decision D51 revised.

backend/internal/server/handlers_yookassa.go

@@ -21,11 +21,22 @@ import (
 // what it read from the provider; this is the backend's own ceiling on an untrusted body.
 const maxNotifyBytes = 1 << 20
 
+// yooKassaReceipt builds the fiscal receipt for a sale, or nil when receipts are off. They are off
+// by default: the merchant runs on НПД, which is outside 54-ФЗ, so nothing is registered through the
+// provider and each operation is reported to «Мой налог» instead. Setting a VAT rate code turns them
+// back on — the switch a lost НПД regime would need — without touching this code.
+func (s *Server) yooKassaReceipt(email, title string, amount yookassa.Amount) *yookassa.Receipt {
+	if !yookassa.ReceiptEnabled(s.vatCode) {
+		return nil
+	}
+	return yookassa.SingleItemReceipt(email, title, amount, s.vatCode)
+}
+
 // orderYooKassa opens a pending order on the direct rail and mints the YooKassa payment the customer
 // is redirected to. shop is the merchant shop the caller resolved for this platform channel, and
-// email is the account's confirmed address (D36), which doubles as the delivery address of the fiscal
-// receipt this rail must send with every payment. No chips are credited here —
-// only later, by the verified notification (or the reconcile sweep).
+// email is the account's confirmed address (D36) — the recovery anchor a direct purchase requires,
+// and the delivery address of a fiscal receipt when receipts are switched on. No chips are credited
+// here — only later, by the verified notification (or the reconcile sweep).
 func (s *Server) orderYooKassa(c *gin.Context, uid uuid.UUID, cxt payments.Context, present []payments.Source, productID uuid.UUID, shop yookassa.Config, email string) {
 	ctx := c.Request.Context()
 	res, err := s.payments.CreateOrder(ctx, uid, cxt, present, productID, providerYooKassa)
@@ -43,9 +54,9 @@ func (s *Server) orderYooKassa(c *gin.Context, uid uuid.UUID, cxt payments.Conte
 		},
 		Description: yookassa.TruncateDescription(res.Title),
 		Metadata:    map[string]string{yookassa.MetadataOrderID: res.OrderID.String()},
-		// «Чеки от ЮKassa»: the receipt is registered only if we send it, so a malformed one is an
-		// API error here rather than a silently missing fiscal document.
-		Receipt: yookassa.SingleItemReceipt(email, res.Title, amount, s.vatCode),
+		// Off unless a VAT rate is configured: the merchant is outside 54-ФЗ and reports each
+		// operation itself, so no fiscal receipt is registered through the provider.
+		Receipt: s.yooKassaReceipt(email, res.Title, amount),
 	}, res.OrderID.String())
 	if err != nil {
 		// The order stays pending and unpaid; it expires on its own. The customer sees the generic
@@ -363,9 +374,9 @@ func isPermanentFundError(err error) bool {
 }
 
 // refundYooKassa returns the money for a paid order through the YooKassa refund API and reports the
-// provider's own refund id, which the ledger then records. The refund carries a receipt because the
-// rail registers a refund receipt the same way it registers a payment one, and the order id as the
-// idempotency key, so a repeated attempt returns the original refund instead of paying twice.
+// provider's own refund id, which the ledger then records. It sends the order id as the idempotency
+// key, so a repeated attempt returns the original refund instead of paying twice, and a refund
+// receipt when receipts are switched on (the rail registers one the same way it does for a payment).
 //
 // It is called before anything is recorded: if the money does not move, nothing is written, and the
 // ledger never claims a refund that did not happen.
@@ -381,15 +392,19 @@ func (s *Server) refundYooKassa(ctx context.Context, ref payments.OrderRef) (str
 	if err != nil {
 		return "", err
 	}
-	email, _, err := s.accounts.ConfirmedEmail(ctx, ref.AccountID)
-	if err != nil {
-		return "", err
+	// The buyer's address is read only to deliver a refund receipt; with receipts off there is
+	// nothing to deliver and nothing to read.
+	email := ""
+	if yookassa.ReceiptEnabled(s.vatCode) {
+		if email, _, err = s.accounts.ConfirmedEmail(ctx, ref.AccountID); err != nil {
+			return "", err
+		}
 	}
 	amount := yookassa.Amount{Value: ref.Amount.Major(), Currency: string(ref.Amount.Currency())}
 	refund, err := shop.CreateRefund(ctx, yookassa.RefundRequest{
 		PaymentID: ref.PaymentID,
 		Amount:    amount,
-		Receipt:   yookassa.SingleItemReceipt(email, title, amount, s.vatCode),
+		Receipt:   s.yooKassaReceipt(email, title, amount),
 	}, ref.OrderID.String())
 	if err != nil {
 		return "", err

backend/internal/server/server.go

@@ -121,6 +121,7 @@ type Deps struct {
 	// Robokassa.
 	YooKassa yookassa.Shops
 	// YooKassaVatCode is the VAT rate code stamped on every fiscal receipt line (54-ФЗ tag 1199).
+	// Unset sends no receipt at all — the state a merchant outside 54-ФЗ runs in.
 	YooKassaVatCode int
 	// PublicBaseURL is the canonical https origin the payment return URL is built from. It is
 	// required whenever YooKassa is configured.