feat(payments): trusted platform signal on the session

Record the execution platform (kind vk|telegram|direct + device subtype
ios|android|web) on each session, captured at creation and carried
gateway->backend as a trusted X-Platform header, so the upcoming
store-compliance gate has an unforgeable execution context.

- backend.sessions gains nullable platform_kind/platform_subtype columns
  (migration 00011, CHECK-constrained, jet regenerated); session.Platform
  captures them at mint, resolve returns them, middleware exposes platform(c).
  kind is derived from the establish endpoint, never a client field; the
  account-merge session mint inherits the caller's platform.
- gateway derives the platform (VK subtype from the signed vk_platform via
  vkauth, Telegram/direct best-effort from the client) and injects X-Platform
  on every authenticated backend call through the request context.
- ui submits a best-effort device subtype on the telegram/guest/email login
  requests (new FBS subtype field); VK is server-derived from the signed params.
- an unattributed session is untrusted (view-only); VK/TG self-heal on the next
  cold-start re-mint, direct/email on re-login.

Signal plumbing only, no user-visible change; X-Platform is inert until the
gate consumes it.

gateway/internal/vkauth/vkauth_test.go

@@ -47,6 +47,33 @@ func TestVerifyValid(t *testing.T) {
 	if id.ExternalID != "494075" || id.Language != "ru" {
 		t.Fatalf("identity = %+v, want ExternalID=494075 Language=ru", id)
 	}
+	if id.Platform != "android" || id.Subtype() != "android" {
+		t.Fatalf("platform = %q subtype = %q, want android/android", id.Platform, id.Subtype())
+	}
+}
+
+// TestSubtype locks the vk_platform -> device-family mapping (the trusted subtype),
+// notably that iPhone and iPad both surface as the store-frozen "ios" and that any
+// unrecognised or empty value falls back to web.
+func TestSubtype(t *testing.T) {
+	for _, tc := range []struct {
+		platform string
+		want     string
+	}{
+		{"mobile_iphone", "ios"},
+		{"mobile_ipad", "ios"},
+		{"mobile_iphone_messenger", "ios"},
+		{"mobile_android", "android"},
+		{"android", "android"},
+		{"desktop_web", "web"},
+		{"mobile_web", "web"},
+		{"", "web"},
+		{"something_new", "web"},
+	} {
+		if got := (vkauth.Identity{Platform: tc.platform}).Subtype(); got != tc.want {
+			t.Errorf("Subtype(%q) = %q, want %q", tc.platform, got, tc.want)
+		}
+	}
 }
 
 // TestVerifyCommaValue locks the URL-encoding of the one realistic special-char VK