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/README.md

@@ -76,10 +76,10 @@ compose binds from this directory.
 | `GM_BASICAUTH_HASH` | secret | bcrypt hash gating `/_gm` (admin console + Grafana). Generate with `docker run --rm caddy:2-alpine caddy hash-password --plaintext '<pw>'`. |
 | `TELEGRAM_MINIAPP_URL` | derived | The Mini App URL the bot hands out in deep links / buttons. The deploy derives `PUBLIC_BASE_URL + /telegram/`; set it directly only for a local run (compose still `:?`-requires it). |
 | `EXPORT_SIGN_KEY` | secret | HMAC key signing the public finished-game export download URLs (`/dl/*`). Generate with `openssl rand -base64 32`. |
-| `BACKEND_ROBOKASSA_MERCHANT_LOGIN` | secret | Robokassa shop login for the direct RUB rail. Empty leaves the rail off. |
-| `BACKEND_ROBOKASSA_PASSWORD1` / `…_PASSWORD2` | secret | Robokassa pass phrases: Password1 signs the launch request, Password2 signs/verifies the Result callback. Use the shop's **test** pair while `…_ROBOKASSA_TEST=1`, the **live** pair for real money. (Password3 — Robokassa's JWT-invoice API — is unused.) |
-| `BACKEND_ROBOKASSA_TEST` | **variable** | `1` runs test payments against the test pass phrases (no real money); empty/`0` is live. A variable, not a secret, so go-live is a flag flip + redeploy, not a secret rotation. |
-| `BACKEND_ROBOKASSA_{WEB,ANDROID}_{MERCHANT_LOGIN,PASSWORD1,PASSWORD2,TEST}` | secret / variable | Multi-shop direct rail (D42): per-channel Robokassa shops. The legacy `BACKEND_ROBOKASSA_*` seeds the **web** shop; `…_WEB_*` overrides it, `…_ANDROID_*` adds the **android** (RuStore) shop. Each credits the one `direct` wallet; the cabinet Result URL per shop is `/pay/robokassa/result/{web,android}`. Empty login ⇒ that channel unconfigured (order routing falls back to the web shop). |
+| `BACKEND_YOOKASSA_{WEB,ANDROID}_SHOP_ID` | secret | YooKassa shop id for that channel of the direct RUB rail (cabinet: **Настройки — Магазин**, field `shopId`). Empty ⇒ that channel is unconfigured and order routing falls back to the **web** shop; no channel configured at all leaves the rail off. |
+| `BACKEND_YOOKASSA_{WEB,ANDROID}_SECRET_KEY` | secret | That shop's API secret key (cabinet: **Интеграция — Ключи API**). It is the password half of the HTTP Basic credentials on every API call. A shop id without a secret key is ignored. |
+| `BACKEND_YOOKASSA_{WEB,ANDROID}_TEST` | **variable** | `1` marks the credentials as a **test shop**: the intake then refuses to credit anything but a test payment (and a live shop refuses a test one). A variable, not a secret, so switching a contour between a test and a live shop is a flag flip + redeploy. |
+| `BACKEND_YOOKASSA_VAT_CODE` | **variable** | VAT rate code stamped on every fiscal receipt line (54-ФЗ tag 1199): `1` = «Без НДС» (УСН/ПСН), `4` = 20%, `11` = 22%. Defaults to `1`. A variable so a rate change needs no code change. **Confirm it with the accountant before the first real payment** — a wrong rate produces wrong receipts, not a failed payment, so it fails silently. |
 
 **Plus the bot token** — `TELEGRAM_BOT_TOKEN` (secret), shared by the validator (HMAC
 secret) and the bot (Bot API). It defaults to empty in compose, but both **fail at