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.

backend/internal/payments/service_intake.go

@@ -46,6 +46,7 @@ func (s *Service) CreateOrder(ctx context.Context, accountID uuid.UUID, cxt Cont
 		amount:    pack.price,
 		origin:    method,
 		provider:  provider,
+		shop:      directShop(cxt),
 	}
 	if err := s.store.createOrder(ctx, o, s.clock()); err != nil {
 		return OrderResult{}, err
@@ -53,6 +54,16 @@ func (s *Service) CreateOrder(ctx context.Context, accountID uuid.UUID, cxt Cont
 	return OrderResult{OrderID: orderID, Amount: pack.price, Title: pack.title}, nil
 }
 
+// directShop returns the merchant shop (channel) a direct order is issued through — the trusted
+// platform subtype for the direct rail ("web"/"android"), or "" for any other rail (the per-shop
+// split is direct-only; D42/D44). Recorded on the order for the admin per-channel breakdown.
+func directShop(cxt Context) string {
+	if cxt.Kind == SourceDirect {
+		return cxt.Subtype
+	}
+	return ""
+}
+
 // OrderItem returns a pending order's human title and the amount it charges, in the order's own
 // currency — the details a provider's item-lookup phase needs (VK's get_item). It reads the order
 // and the pack title, honouring the pack even if it was later deactivated (mirrors Fund).