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.

gateway/internal/backendclient/api.go

@@ -514,6 +514,16 @@ func (c *Client) RobokassaResult(ctx context.Context, channel string, params map
 	return out.Response, err
 }
 
+// YooKassaNotify forwards a YooKassa webhook body verbatim to the backend intake (the single writer,
+// which re-reads the payment from the provider before acting on it — YooKassa does not sign
+// notifications). The backend answers 200 for anything durably decided and 5xx only for a transient
+// failure, so a nil error means the provider may stop redelivering.
+// clientIP is the true sender address, which only the gateway sees; the backend checks it against
+// YooKassa's published sender ranges.
+func (c *Client) YooKassaNotify(ctx context.Context, clientIP string, body json.RawMessage) error {
+	return c.do(ctx, http.MethodPost, "/api/v1/internal/payments/yookassa/notify", "", clientIP, body, nil)
+}
+
 // VKCallback forwards a gateway-verified VK payment callback's parameters to the backend intake and
 // returns the backend's raw VK response envelope (`{"response":…}` or `{"error":…}`) for the gateway
 // to relay to VK verbatim. The backend answers 200 in every case (VK's protocol), so a transport