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.

docs/PAYMENTS.md

@@ -57,6 +57,15 @@ for outside the store's own cash desk. Chips funded inside VK (Votes) may only b
 the VK context; Stars only inside Telegram; Robokassa-funded (`direct`) chips only outside
 the stores. See §4.
 
+**Multi-shop direct rail (D42).** The `direct` rail routes to one Robokassa **merchant shop per
+channel** — `web` and `android` (RuStore), `ios` later — chosen by the trusted `X-Platform` subtype;
+every shop credits the one `direct` wallet (no per-channel wallet). This is merchant-account
+separation for accounting / receipts only: the order records its `shop` (shown in the admin report),
+and each shop's Result callback is verified by its own Password2 at `/pay/robokassa/result/<channel>`.
+Standalone apps (Android/iOS) sign in by email only, so a direct purchase always has the D36 email
+anchor (D43). Fiscalization (54-ФЗ via the Robokassa cabinet under one ИП) is a single source
+regardless of the number of shops (D41).
+
 ## 3. Three operations, kept distinct
 
 Do not conflate these — they use different keys: