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.

pkg/fbs/scrabble.fbs

@@ -106,6 +106,9 @@ table MoveRecord {
 table TelegramLoginRequest {
   init_data:string;
   browser_tz:string;
+  // Client-reported device family (ios/android/web). Telegram's initData does not sign it, so
+  // the server records it best-effort and the payments gate never relies on it.
+  subtype:string;
 }
 
 // VKLoginRequest carries a VK Mini App launch. params is the raw query string of the
@@ -128,6 +131,9 @@ table VKLoginRequest {
 table GuestLoginRequest {
   locale:string;
   browser_tz:string;
+  // Client-reported device family (ios/android/web) of this direct session; best-effort
+  // (a direct session has no external signer).
+  subtype:string;
 }
 
 // EmailRequestRequest asks the backend to send a login confirm-code to email. It
@@ -150,6 +156,8 @@ table EmailRequestRequest {
 table EmailLoginRequest {
   email:string;
   code:string;
+  // Client-reported device family (ios/android/web) of this direct session; best-effort.
+  subtype:string;
 }
 
 // EmailConfirmLinkRequest verifies a one-tap deeplink token from a confirmation