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/docker-compose.yml

@@ -171,6 +171,18 @@ services:
       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:-}
     # 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