feat(account): deletion orchestration + step-up + gateway edge

Step-up: email accounts confirm with a mailed code (purpose=delete, no deeplink —
ConfirmByToken refuses a delete token so a stray click can't delete); platform-only
accounts type a fixed phrase (anti-impulse). Endpoints /user/delete/{request,confirm};
the confirm orchestration resigns active games, drops all-robot games, tombstones +
anonymizes the account (freeing its creds), and revokes its sessions — the tombstone is
the point of no return, the rest best-effort. Gateway account.delete.{request,confirm}
ops + fbs AccountDeleteConfirm/AccountDeleteRequestResult + branded ru/en delete email.
Integration tests cover the step-up (code + no-email) and the orchestration pieces.

gateway/internal/transcode/encode.go

@@ -37,6 +37,17 @@ func encodeAck(ok bool) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeDeleteRequestResult builds an AccountDeleteRequestResult payload reporting which
+// deletion step-up the account uses ("email" | "phrase").
+func encodeDeleteRequestResult(method string) []byte {
+	b := flatbuffers.NewBuilder(32)
+	m := b.CreateString(method)
+	fb.AccountDeleteRequestResultStart(b)
+	fb.AccountDeleteRequestResultAddMethod(b, m)
+	b.Finish(fb.AccountDeleteRequestResultEnd(b))
+	return b.FinishedBytes()
+}
+
 // encodeConfirmLinkResult builds an EmailConfirmLinkResult payload, embedding the
 // minted Session for a login. All strings and the nested Session table are built
 // before the result table is opened.