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.

backend/internal/inttest/session_test.go

@@ -26,7 +26,8 @@ func TestSessionLifecycle(t *testing.T) {
 	store := session.NewStore(testDB)
 	svc := session.NewService(store, session.NewCache())
 
-	token, sess, err := svc.Create(ctx, acc.ID)
+	platform := session.Platform{Kind: session.PlatformKindVK, Subtype: session.SubtypeIOS}
+	token, sess, err := svc.Create(ctx, acc.ID, platform)
 	if err != nil {
 		t.Fatalf("create session: %v", err)
 	}
@@ -36,6 +37,9 @@ func TestSessionLifecycle(t *testing.T) {
 	if token == sess.TokenHash {
 		t.Error("plaintext token must not equal the stored hash")
 	}
+	if sess.Platform != platform {
+		t.Errorf("session platform = %+v, want %+v", sess.Platform, platform)
+	}
 
 	// Resolve via the warm write-through cache.
 	got, err := svc.Resolve(ctx, token)
@@ -63,8 +67,8 @@ func TestSessionLifecycle(t *testing.T) {
 	if _, ok := cold.Get(session.HashToken(token)); !ok {
 		t.Error("Warm must load the active session into the cache")
 	}
-	if got2, err := svc2.Resolve(ctx, token); err != nil || got2.ID != sess.ID {
-		t.Errorf("resolve after warm = (%s, %v), want %s", got2.ID, err, sess.ID)
+	if got2, err := svc2.Resolve(ctx, token); err != nil || got2.ID != sess.ID || got2.Platform != platform {
+		t.Errorf("resolve after warm = (%s, %+v, %v), want %s / %+v", got2.ID, got2.Platform, err, sess.ID, platform)
 	}
 
 	// Revoke, then the token no longer resolves; revoke again is a no-op.