feat(payments): per-channel Robokassa shops on the direct rail

Split the single Robokassa direct rail into one merchant shop per channel
(web / android; ios later), chosen by the trusted X-Platform subtype, while
every shop still credits the one `direct` wallet — merchant-account separation
for accounting and receipts, not a new wallet.

- config: a channel-keyed shops registry (web seeds from the legacy vars or
  _WEB_*, android from _ANDROID_*); an empty shop leaves the rail dormant;
  per-shop validation.
- intake: the order picks its shop by subtype (unknown falls back to web); the
  per-shop Result callback is verified by that shop's own Password2 at
  /pay/robokassa/result/<channel> (the gateway extracts the channel; Caddy's
  /pay/* glob already forwards it — no Caddyfile change).
- persistence: an additive `shop` column on the order (migration 00015),
  recorded from the payment context, surfaced per entry in the admin report.
- standalone apps sign in by email only, so a direct purchase keeps its email
  anchor.
- docs: PAYMENTS (+ru) topology, deploy env vars + compose mapping, the
  decisions log (D41 revised for the ИП / 54-ФЗ move; D42-D44), the plan.

Contour-safe: dormant until shops are configured; the migration is additive
(no wipe); no client wire change. Fiscalization (Receipt/Email) and the gateway
`direct/android` subtype follow when the ИП / RuStore are live.

deploy/README.md

@@ -79,6 +79,7 @@ compose binds from this directory.
 | `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). |
 
 **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