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/.env.example

@@ -132,28 +132,30 @@ GATEWAY_VK_APP_SECRET=
 # come from VITE_VK_APP_ID / VITE_VK_ID_REDIRECT_URL above. All three empty disables link.vk.*.
 GATEWAY_VK_ID_CLIENT_SECRET=
 
-# --- Payments: Robokassa (direct RUB rail) ----------------------------------
-# The shop's merchant login + the two pass phrases (Password1 signs the launch request,
-# Password2 signs/verifies the Result callback). An empty login leaves the direct rail off.
-# ROBOKASSA_TEST=1 runs test payments against the shop's TEST pass phrases (no real money);
-# empty/0 is live. Mapped in compose to BACKEND_ROBOKASSA_*; Gitea TEST_/PROD_ secrets, with
-# PROD_BACKEND_ROBOKASSA_TEST a variable so go-live is a flag flip, not a secret redeploy.
-ROBOKASSA_MERCHANT_LOGIN=
-ROBOKASSA_PASSWORD1=
-ROBOKASSA_PASSWORD2=
-ROBOKASSA_TEST=
-# Multi-shop direct rail (D42): the vars above seed the "web" channel; add per-channel shops here.
-# ROBOKASSA_WEB_* overrides the web shop; ROBOKASSA_ANDROID_* adds the android (RuStore) shop. Each
-# credits the one direct wallet; an empty login drops that channel (routing falls back to web). The
-# Robokassa cabinet points each shop's Result URL at /pay/robokassa/result/web and .../android.
-ROBOKASSA_WEB_MERCHANT_LOGIN=
-ROBOKASSA_WEB_PASSWORD1=
-ROBOKASSA_WEB_PASSWORD2=
-ROBOKASSA_WEB_TEST=
-ROBOKASSA_ANDROID_MERCHANT_LOGIN=
-ROBOKASSA_ANDROID_PASSWORD1=
-ROBOKASSA_ANDROID_PASSWORD2=
-ROBOKASSA_ANDROID_TEST=
+# --- Payments: YooKassa (direct RUB rail) ------------------------------------
+# One merchant shop per channel (D42): the shop id (Настройки — Магазин, field shopId) and the
+# secret key (Интеграция — Ключи API). Each shop credits the one direct wallet; a shop missing
+# either credential drops that channel (order routing falls back to web), and no shop at all
+# leaves the direct rail off. YOOKASSA_*_TEST=1 marks a TEST shop, which makes the intake refuse
+# to credit a live payment against it (and a test payment against a live shop).
+# Mapped in compose to BACKEND_YOOKASSA_*; Gitea TEST_/PROD_ secrets.
+# Set each shop's notification URL in its cabinet (Интеграция — HTTP-уведомления) to
+# ${PUBLIC_BASE_URL}/pay/yookassa/notify, events payment.succeeded + payment.canceled.
+YOOKASSA_WEB_SHOP_ID=
+YOOKASSA_WEB_SECRET_KEY=
+YOOKASSA_WEB_TEST=
+# The android (RuStore) shop — add when that channel gets its own merchant account.
+YOOKASSA_ANDROID_SHOP_ID=
+YOOKASSA_ANDROID_SECRET_KEY=
+YOOKASSA_ANDROID_TEST=
+# VAT rate code on every fiscal receipt line (54-ФЗ tag 1199): 1 = «Без НДС» (УСН/ПСН), 4 = 20%,
+# 11 = 22%. A Gitea VARIABLE, not a secret, so a rate change is a variable edit + redeploy.
+YOOKASSA_VAT_CODE=1
+#
+# Robokassa (the retired direct rail) is deliberately absent here: no deployment sets its
+# credentials, which is what keeps it dormant. The backend still reads BACKEND_ROBOKASSA_* and
+# falls the direct rail back to it if they are ever set again — see
+# backend/internal/robokassa/README.md for the full variable set and how to revive the rail.
 
 # --- Gateway anti-abuse ------------------------------------------------------
 # Planted honeytoken bearer value: any request presenting it is flagged — a 24h IP ban