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/statement.go

@@ -42,7 +42,8 @@ type RiskInfo struct {
 }
 
 // LedgerEntry is one append-only ledger row projected for the report. The string ids are empty when
-// the column is NULL; Snapshot is the raw purchase/refund JSON (empty when none).
+// the column is NULL; Shop is the direct-rail merchant channel the referenced order used (empty for
+// other rails or order-less rows); Snapshot is the raw purchase/refund JSON (empty when none).
 type LedgerEntry struct {
 	Kind              string
 	Source            string
@@ -52,6 +53,7 @@ type LedgerEntry struct {
 	OrderID           string
 	Provider          string
 	ProviderPaymentID string
+	Shop              string
 	Snapshot          string
 	CreatedAt         time.Time
 }