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.go

@@ -22,10 +22,28 @@ var ErrInvalid = errors.New("vkauth: invalid vk launch params")
 
 // Identity is the user extracted from verified VK launch params. ExternalID is the
 // vk_user_id used as the identities external_id; Language is the vk_language hint that
-// seeds a brand-new account's preferred language.
+// seeds a brand-new account's preferred language; Platform is the raw signed
+// vk_platform value (e.g. "mobile_iphone", "desktop_web"), from which Subtype derives
+// the trusted device family.
 type Identity struct {
 	ExternalID string
 	Language   string
+	Platform   string
+}
+
+// Subtype maps the signed vk_platform to the trusted device family recorded on the
+// session — ios (iPhone/iPad, the store-frozen case), android, or web (desktop/mobile
+// web and anything unrecognised). Because vk_platform rides inside the verified
+// signature, this subtype is trustworthy, unlike a client-reported one.
+func (i Identity) Subtype() string {
+	switch {
+	case strings.Contains(i.Platform, "android"):
+		return "android"
+	case strings.Contains(i.Platform, "iphone"), strings.Contains(i.Platform, "ipad"):
+		return "ios"
+	default:
+		return "web"
+	}
 }
 
 // Verify checks the `sign` of a VK Mini App launch query string against secret and
@@ -68,5 +86,5 @@ func Verify(params, secret string) (Identity, error) {
 	if externalID == "" {
 		return Identity{}, ErrInvalid
 	}
-	return Identity{ExternalID: externalID, Language: values.Get("vk_language")}, nil
+	return Identity{ExternalID: externalID, Language: values.Get("vk_language"), Platform: values.Get("vk_platform")}, nil
 }