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/postgres/jet/payments/model/orders.go

@@ -25,4 +25,5 @@ type Orders struct {
 	ProviderPaymentID *string
 	CreatedAt         time.Time
 	UpdatedAt         time.Time
+	Shop              string
 }

backend/internal/postgres/jet/payments/table/orders.go

@@ -29,6 +29,7 @@ type ordersTable struct {
 	ProviderPaymentID postgres.ColumnString
 	CreatedAt         postgres.ColumnTimestampz
 	UpdatedAt         postgres.ColumnTimestampz
+	Shop              postgres.ColumnString
 
 	AllColumns     postgres.ColumnList
 	MutableColumns postgres.ColumnList
@@ -82,9 +83,10 @@ func newOrdersTableImpl(schemaName, tableName, alias string) ordersTable {
 		ProviderPaymentIDColumn = postgres.StringColumn("provider_payment_id")
 		CreatedAtColumn         = postgres.TimestampzColumn("created_at")
 		UpdatedAtColumn         = postgres.TimestampzColumn("updated_at")
-		allColumns              = postgres.ColumnList{OrderIDColumn, AccountIDColumn, PlatformColumn, ProductIDColumn, ExpectedAmountColumn, CurrencyColumn, OriginColumn, StatusColumn, ProviderColumn, ProviderPaymentIDColumn, CreatedAtColumn, UpdatedAtColumn}
-		mutableColumns          = postgres.ColumnList{AccountIDColumn, PlatformColumn, ProductIDColumn, ExpectedAmountColumn, CurrencyColumn, OriginColumn, StatusColumn, ProviderColumn, ProviderPaymentIDColumn, CreatedAtColumn, UpdatedAtColumn}
-		defaultColumns          = postgres.ColumnList{StatusColumn, CreatedAtColumn, UpdatedAtColumn}
+		ShopColumn              = postgres.StringColumn("shop")
+		allColumns              = postgres.ColumnList{OrderIDColumn, AccountIDColumn, PlatformColumn, ProductIDColumn, ExpectedAmountColumn, CurrencyColumn, OriginColumn, StatusColumn, ProviderColumn, ProviderPaymentIDColumn, CreatedAtColumn, UpdatedAtColumn, ShopColumn}
+		mutableColumns          = postgres.ColumnList{AccountIDColumn, PlatformColumn, ProductIDColumn, ExpectedAmountColumn, CurrencyColumn, OriginColumn, StatusColumn, ProviderColumn, ProviderPaymentIDColumn, CreatedAtColumn, UpdatedAtColumn, ShopColumn}
+		defaultColumns          = postgres.ColumnList{StatusColumn, CreatedAtColumn, UpdatedAtColumn, ShopColumn}
 	)
 
 	return ordersTable{
@@ -103,6 +105,7 @@ func newOrdersTableImpl(schemaName, tableName, alias string) ordersTable {
 		ProviderPaymentID: ProviderPaymentIDColumn,
 		CreatedAt:         CreatedAtColumn,
 		UpdatedAt:         UpdatedAtColumn,
+		Shop:              ShopColumn,
 
 		AllColumns:     allColumns,
 		MutableColumns: mutableColumns,