feat(payments): settle the direct rail through YooKassa

Replace Robokassa with YooKassa as the RUB direct-rail provider. The wallet
model is untouched: one `direct` segment, the same spend wall, the same
per-channel merchant shops (D42) and `shop` on the order (D44).

The two providers are not shaped alike, and that drives the change:

- Opening a purchase is now an outbound API call (`POST /v3/payments`,
  single-stage capture, redirect confirmation). The order id is both the
  `Idempotence-Key` and `metadata.order_id`, so a retried create cannot mint a
  second payment and a notification always resolves to its order.
- YooKassa does NOT sign notifications, so the body is never evidence: it only
  names a payment, which is re-read with `GET /v3/payments/{id}`, and only that
  answer is acted on. Two guards ride on it — the payment's metadata must name
  the order, and its `test` flag must match the shop's, so a test-shop payment
  can never credit real chips. The sender address is checked against YooKassa's
  published ranges first, which stops a forger turning each fabricated
  notification into an outbound call of ours.
- A notification lost for good would leave the money taken and the chips unowed,
  silently. The existing pending-order reaper now asks the provider about each
  order that reached its expiry age carrying a payment id, and credits the ones
  really paid — one request per order over its whole life, not polling.
- `payment.canceled` records a `failed` event, so a declined payment is finally
  surfaced to the customer as PAYMENTS.md §9 already specified.
- The admin refund moves the money through `POST /v3/refunds` before recording
  anything; a failed call records nothing, so the ledger cannot claim a refund
  that did not happen, and the recorded id is the provider's own.
- YooKassa has no cabinet-side generic receipt: «Чеки от ЮKassa» registers one
  only if the request carries it, so every payment and refund now sends an
  itemized `receipt` to the D36 confirmed email. The VAT rate code is a deploy
  variable; the settlement subject and method are constants.

Robokassa is retired, not deleted: the direct rail falls back to it when no
YooKassa shop is configured and no deployment sets its credentials, so reviving
it is a credentials change rather than a code change. Its variables are removed
from compose, .env.example, write-prod-env.sh and the three workflows, and
recorded in backend/internal/robokassa/README.md together with the cabinet
configuration and the revival steps. Ledger rows keep `provider = 'robokassa'`;
that literal is load-bearing for the idempotency index.

No migration and no wire change: `orders.provider_payment_id` already existed,
and the client is rail-agnostic.

Decisions D47-D51 (revising D41) and stage E12 are baked into the docs.

backend/internal/payments/refund_admin.go

@@ -11,11 +11,24 @@ import (
 // one manual full refund per order.
 const providerAdmin = "admin"
 
-// RefundOrderFull refunds a paid order in full at the operator's request: it revokes the funded
-// chips best-effort (floored at 0, never negative — D27), records a refund ledger row, and is
-// idempotent (a second call reports AlreadyRefunded). The operator performs the actual money refund
-// on the rail (Robokassa cabinet / VK support / Telegram refundStarPayment); this records it.
+// RefundOrderFull refunds a paid order in full at the operator's request, recording the reversal
+// only. It is for the rails that have no refund API of our own to call: the operator performs the
+// actual money refund there by hand (VK support, Telegram refundStarPayment, the Robokassa cabinet),
+// and this records it under the operator's own idempotency key.
 func (s *Service) RefundOrderFull(ctx context.Context, orderID uuid.UUID) (RefundOutcome, error) {
+	return s.RefundOrderFullAs(ctx, orderID, providerAdmin, orderID.String())
+}
+
+// RefundOrderFullAs refunds a paid order in full and records it under the given provider and refund
+// id: it revokes the funded chips best-effort (floored at 0, never negative — D27), appends a refund
+// ledger row and is idempotent on (provider, providerRefundID), so a second call reports
+// AlreadyRefunded. Callers whose rail moved the money through an API pass that rail's own refund id,
+// which keeps the ledger reconcilable against the provider's records; the refund id is distinct from
+// the fund's payment id, so the two rows coexist under the same partial-unique index.
+//
+// The provider-side money movement must already have succeeded when this is called — the ledger must
+// never claim a refund that did not happen.
+func (s *Service) RefundOrderFullAs(ctx context.Context, orderID uuid.UUID, provider, providerRefundID string) (RefundOutcome, error) {
 	o, err := s.store.orderByID(ctx, orderID)
 	if err != nil {
 		return RefundOutcome{}, err
@@ -24,7 +37,7 @@ func (s *Service) RefundOrderFull(ctx context.Context, orderID uuid.UUID) (Refun
 	if err != nil {
 		return RefundOutcome{}, err
 	}
-	return s.store.refund(ctx, orderID, providerAdmin, orderID.String(), refunded, s.clock())
+	return s.store.refund(ctx, orderID, provider, providerRefundID, refunded, s.clock())
 }
 
 // LedgerExportRow is one append-only ledger row for the tax / reconciliation export, carrying the