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/server/handlers_admin_console.go

@@ -482,7 +482,7 @@ func financeView(stmt payments.Statement) adminconsole.FinanceView {
 	for _, e := range stmt.Ledger {
 		fv.Ledger = append(fv.Ledger, adminconsole.LedgerRow{
 			Kind: e.Kind, Source: e.Source, Origin: e.Origin, ChipsDelta: e.ChipsDelta,
-			Product: e.ProductID, Order: e.OrderID, Provider: e.Provider, Snapshot: e.Snapshot,
+			Product: e.ProductID, Order: e.OrderID, Provider: e.Provider, Shop: e.Shop, Snapshot: e.Snapshot,
 			At: fmtTime(e.CreatedAt),
 		})
 	}