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.

gateway/internal/connectsrv/robokassa_channel_test.go

@@ -0,0 +1,17 @@
+package connectsrv
+
+import "testing"
+
+func TestRobokassaChannel(t *testing.T) {
+	cases := map[string]string{
+		"/pay/robokassa/result":         "", // legacy bare path → the backend defaults to the web shop
+		"/pay/robokassa/result/":        "", // trailing slash, no channel → the web default
+		"/pay/robokassa/result/web":     "web",
+		"/pay/robokassa/result/android": "android",
+	}
+	for path, want := range cases {
+		if got := robokassaChannel(path); got != want {
+			t.Errorf("robokassaChannel(%q) = %q, want %q", path, got, want)
+		}
+	}
+}