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/config/config.go

@@ -65,9 +65,9 @@ type Config struct {
 	// RendererURL is the base URL of the internal image-render sidecar (e.g.
 	// http://renderer:8090). Empty disables the PNG export artifact.
 	RendererURL string
-	// Robokassa configures the direct-rail (RUB) payment provider. An empty MerchantLogin
-	// leaves the direct order and Result-callback endpoints unregistered.
-	Robokassa robokassa.Config
+	// Robokassa configures the direct-rail (RUB) payment provider — one merchant shop per channel
+	// (D42). An empty set leaves the direct order and Result-callback endpoints unregistered.
+	Robokassa robokassa.Shops
 }
 
 // Defaults applied when the corresponding environment variable is unset.
@@ -157,11 +157,19 @@ func Load() (Config, error) {
 		AdminTo:   os.Getenv("BACKEND_ADMIN_EMAIL"),
 	}
 
-	robo := robokassa.Config{
-		MerchantLogin: os.Getenv("BACKEND_ROBOKASSA_MERCHANT_LOGIN"),
-		Password1:     os.Getenv("BACKEND_ROBOKASSA_PASSWORD1"),
-		Password2:     os.Getenv("BACKEND_ROBOKASSA_PASSWORD2"),
-		IsTest:        os.Getenv("BACKEND_ROBOKASSA_TEST") == "1",
+	// Robokassa direct rail: one merchant shop per channel (D42). The legacy single-shop vars seed
+	// the web channel so existing deploys keep working; the per-channel vars add the rest. A shop
+	// with no MerchantLogin is dropped (the rail stays dormant when none is configured).
+	shops := robokassa.Shops{}
+	web := robokassaShop("BACKEND_ROBOKASSA_WEB")
+	if web.MerchantLogin == "" {
+		web = robokassaShop("BACKEND_ROBOKASSA") // legacy single-shop credentials seed the web channel
+	}
+	if web.MerchantLogin != "" {
+		shops[robokassa.ChannelWeb] = web
+	}
+	if android := robokassaShop("BACKEND_ROBOKASSA_ANDROID"); android.MerchantLogin != "" {
+		shops[robokassa.ChannelAndroid] = android
 	}
 
 	c := Config{
@@ -181,7 +189,7 @@ func Load() (Config, error) {
 		GuestRetention:    guestRetention,
 		ExportSignKey:     os.Getenv("BACKEND_EXPORT_SIGN_KEY"),
 		RendererURL:       os.Getenv("BACKEND_RENDERER_URL"),
-		Robokassa:         robo,
+		Robokassa:         shops,
 	}
 	if err := c.validate(); err != nil {
 		return Config{}, err
@@ -234,8 +242,10 @@ func (c Config) validate() error {
 			return fmt.Errorf("config: BACKEND_PUBLIC_BASE_URL %q must be an absolute URL (scheme://host)", c.PublicBaseURL)
 		}
 	}
-	if c.Robokassa.MerchantLogin != "" && (c.Robokassa.Password1 == "" || c.Robokassa.Password2 == "") {
-		return fmt.Errorf("config: BACKEND_ROBOKASSA_PASSWORD1 and BACKEND_ROBOKASSA_PASSWORD2 must be set when BACKEND_ROBOKASSA_MERCHANT_LOGIN is")
+	for channel, shop := range c.Robokassa {
+		if shop.Password1 == "" || shop.Password2 == "" {
+			return fmt.Errorf("config: robokassa shop %q: password1 and password2 must be set when its merchant login is", channel)
+		}
 	}
 	return nil
 }
@@ -276,3 +286,15 @@ func envDuration(key string, fallback time.Duration) (time.Duration, error) {
 	}
 	return d, nil
 }
+
+// robokassaShop reads a Robokassa shop's four credentials from the environment under prefix (e.g.
+// "BACKEND_ROBOKASSA_WEB" → _MERCHANT_LOGIN / _PASSWORD1 / _PASSWORD2 / _TEST). A missing
+// MerchantLogin yields a zero Config the caller drops.
+func robokassaShop(prefix string) robokassa.Config {
+	return robokassa.Config{
+		MerchantLogin: os.Getenv(prefix + "_MERCHANT_LOGIN"),
+		Password1:     os.Getenv(prefix + "_PASSWORD1"),
+		Password2:     os.Getenv(prefix + "_PASSWORD2"),
+		IsTest:        os.Getenv(prefix+"_TEST") == "1",
+	}
+}