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/robokassa/shops.go

@@ -0,0 +1,54 @@
+package robokassa
+
+// Shops is a set of Robokassa merchant shops keyed by channel — the subtype of the trusted
+// X-Platform signal for the direct rail ("web", "android"; "ios" later). The direct rail routes a
+// payment to the per-channel shop for separate merchant accounts, accounting and receipts, while
+// every shop still credits the one direct wallet (docs/PAYMENTS_DECISIONS_ru.md D42). An empty set
+// leaves the direct order and Result-callback endpoints unregistered.
+type Shops map[string]Config
+
+// Channel constants name the direct-rail X-Platform subtypes a shop is keyed by. ChannelWeb is the
+// default: an unknown or empty channel routes here on the order side, and the legacy single-shop
+// configuration seeds it.
+const (
+	ChannelWeb     = "web"
+	ChannelAndroid = "android"
+)
+
+// Shop returns the shop that issues an order for channel, falling back to the web shop when channel
+// is unknown, empty or not configured. The fallback is safe: routing only chooses the merchant
+// account and receipt, never the credited wallet (always direct, D42), so a mis-attributed channel
+// costs at most accounting accuracy, not money. The second result is false when neither the channel
+// nor the web shop has a merchant login (the rail is unconfigured).
+func (s Shops) Shop(channel string) (Config, bool) {
+	if channel != "" {
+		if c, ok := s[channel]; ok && c.MerchantLogin != "" {
+			return c, true
+		}
+	}
+	if c, ok := s[ChannelWeb]; ok && c.MerchantLogin != "" {
+		return c, true
+	}
+	return Config{}, false
+}
+
+// Verifier returns the shop whose Password2 verifies a Result callback delivered to channel's
+// dedicated Result route. Unlike Shop it does not fall back to web: each shop's callback is verified
+// only by that shop's own credentials, so a route with no configured shop reports false (and its
+// handler answers as unregistered). The second result is false when channel has no merchant login.
+func (s Shops) Verifier(channel string) (Config, bool) {
+	if c, ok := s[channel]; ok && c.MerchantLogin != "" {
+		return c, true
+	}
+	return Config{}, false
+}
+
+// Configured reports whether at least one shop has a merchant login (the direct rail is live).
+func (s Shops) Configured() bool {
+	for _, c := range s {
+		if c.MerchantLogin != "" {
+			return true
+		}
+	}
+	return false
+}