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/.env.example

@@ -142,6 +142,18 @@ 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=
 
 # --- Gateway anti-abuse ------------------------------------------------------
 # Planted honeytoken bearer value: any request presenting it is flagged — a 24h IP ban