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.

deploy/docker-compose.yml

@@ -164,25 +164,21 @@ services:
       # recipient(s) (comma-separated allowed). Both empty disables the alert worker.
       BACKEND_SMTP_ADMIN_FROM: ${SMTP_RELAY_ADMIN_FROM:-}
       BACKEND_ADMIN_EMAIL: ${ADMIN_EMAIL:-}
-      # Direct-rail (Robokassa) payment intake: the merchant login + Password1/Password2 and the
-      # test-mode flag (deploy env: TEST_/PROD_BACKEND_ROBOKASSA_*). An empty login leaves the
-      # direct order and Result-callback endpoints unregistered (the rail is off).
-      BACKEND_ROBOKASSA_MERCHANT_LOGIN: ${ROBOKASSA_MERCHANT_LOGIN:-}
-      BACKEND_ROBOKASSA_PASSWORD1: ${ROBOKASSA_PASSWORD1:-}
-      BACKEND_ROBOKASSA_PASSWORD2: ${ROBOKASSA_PASSWORD2:-}
-      BACKEND_ROBOKASSA_TEST: ${ROBOKASSA_TEST:-}
-      # Per-channel shops for the multi-shop direct rail (D42): the legacy ROBOKASSA_* above seeds
-      # the "web" channel; ROBOKASSA_WEB_* overrides it and ROBOKASSA_ANDROID_* adds the android
-      # shop. Each shop credits the one direct wallet; an empty login drops that channel (order
-      # routing falls back to the web shop). Result URLs: /pay/robokassa/result/{web,android}.
-      BACKEND_ROBOKASSA_WEB_MERCHANT_LOGIN: ${ROBOKASSA_WEB_MERCHANT_LOGIN:-}
-      BACKEND_ROBOKASSA_WEB_PASSWORD1: ${ROBOKASSA_WEB_PASSWORD1:-}
-      BACKEND_ROBOKASSA_WEB_PASSWORD2: ${ROBOKASSA_WEB_PASSWORD2:-}
-      BACKEND_ROBOKASSA_WEB_TEST: ${ROBOKASSA_WEB_TEST:-}
-      BACKEND_ROBOKASSA_ANDROID_MERCHANT_LOGIN: ${ROBOKASSA_ANDROID_MERCHANT_LOGIN:-}
-      BACKEND_ROBOKASSA_ANDROID_PASSWORD1: ${ROBOKASSA_ANDROID_PASSWORD1:-}
-      BACKEND_ROBOKASSA_ANDROID_PASSWORD2: ${ROBOKASSA_ANDROID_PASSWORD2:-}
-      BACKEND_ROBOKASSA_ANDROID_TEST: ${ROBOKASSA_ANDROID_TEST:-}
+      # Direct-rail (YooKassa) payment intake, one merchant shop per channel (D42): the shop id +
+      # secret key and a flag marking a test shop (deploy env: TEST_/PROD_BACKEND_YOOKASSA_*).
+      # Each shop credits the one direct wallet; a shop missing either credential drops that
+      # channel (order routing falls back to the web shop), and no shop at all leaves the direct
+      # order and notification endpoints unregistered. Notification URL (set per shop in the
+      # YooKassa cabinet): ${PUBLIC_BASE_URL}/pay/yookassa/notify.
+      BACKEND_YOOKASSA_WEB_SHOP_ID: ${YOOKASSA_WEB_SHOP_ID:-}
+      BACKEND_YOOKASSA_WEB_SECRET_KEY: ${YOOKASSA_WEB_SECRET_KEY:-}
+      BACKEND_YOOKASSA_WEB_TEST: ${YOOKASSA_WEB_TEST:-}
+      BACKEND_YOOKASSA_ANDROID_SHOP_ID: ${YOOKASSA_ANDROID_SHOP_ID:-}
+      BACKEND_YOOKASSA_ANDROID_SECRET_KEY: ${YOOKASSA_ANDROID_SECRET_KEY:-}
+      BACKEND_YOOKASSA_ANDROID_TEST: ${YOOKASSA_ANDROID_TEST:-}
+      # VAT rate code stamped on every fiscal receipt line (54-ФЗ tag 1199). 1 = «Без НДС» (УСН/ПСН).
+      # A deploy variable, not a secret, so a rate change is a variable edit + redeploy.
+      BACKEND_YOOKASSA_VAT_CODE: ${YOOKASSA_VAT_CODE:-1}
     # The dictionary lives on a named volume seeded from the image on first boot
     # (the image's /opt/dawg is owned by the nonroot UID, which the fresh volume
     # inherits). The admin console writes new version subdirectories here, and the