feat(telegram): split connector into home validator + remote bot

Move all Telegram egress off the main host. The single connector held the
bot token, long-polled Telegram and answered the gateway/backend over the
trusted internal network, so the whole component (including login validation)
shared fate with its VPN sidecar. Split it into two binaries that share the
token:

- cmd/validator (home, no VPN): Mini App initData + Login Widget HMAC only,
  never calls the Bot API. The gateway dials it for Telegram auth, so game
  login is now independent of Telegram reachability.
- cmd/bot (remote): Bot API long-poll + sendMessage, the only component
  reaching Telegram. It holds no inbound port — it dials the gateway over a
  new reverse mTLS bot-link (pkg/proto/botlink/v1) and executes the send
  commands the gateway pushes.

The gateway funnels sends to the bot-link: out-of-app push is fire-and-forget
(at-most-once, dropped if no bot is connected); the backend admin broadcasts
reach a gateway-served relay that forwards them and awaits the bot's ack
(SendToUser/SendToGameChannel contract preserved). mTLS (pkg/mtls) is the one
inter-service link that leaves the trusted segment; validator<->gateway and
the relay stay plaintext internal. The bot is Telegram-rate-limited.

One bot now; the gateway bot registry, an owns_updates flag and per-command
ids leave seams for N later. Webhook rejected (one URL per token, adds inbound
+ a static address).

The unified test contour runs the split (the bot keeps its VPN sidecar and
dials the gateway by its internal name; bot-link certs from deploy/gen-certs.sh,
generated in CI). The prod wiring — the bot on a separate host (no VPN), the
gateway bot-link port published, PROD_ certs with scheduled rotation, an SSH
deploy of both hosts together — is the deferred final stage (PRERELEASE.md TX,
Stage 18).

Docs: ARCHITECTURE, PRERELEASE (phase TX), platform/telegram + gateway +
backend + deploy READMEs, FUNCTIONAL(+ru), CLAUDE.md, .env.example.

gateway/internal/botlink/relay.go

@@ -0,0 +1,67 @@
+package botlink
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+
+	botlinkv1 "scrabble/pkg/proto/botlink/v1"
+	telegramv1 "scrabble/pkg/proto/telegram/v1"
+)
+
+// RelayServer lets the backend's admin console reach the remote bot through the
+// gateway: it implements the two admin delivery methods of the Telegram service by
+// forwarding them onto the bot-link and awaiting the bot's Ack. The validation and
+// out-of-app push methods are intentionally unimplemented here — login validation
+// runs against the home validator and out-of-app push goes straight to the Hub.
+type RelayServer struct {
+	telegramv1.UnimplementedTelegramServer
+	hub      *Hub
+	deadline time.Duration
+}
+
+// NewRelayServer builds the relay over hub. deadline bounds the wait for the bot's
+// Ack before reporting the send as not delivered.
+func NewRelayServer(hub *Hub, deadline time.Duration) *RelayServer {
+	return &RelayServer{hub: hub, deadline: deadline}
+}
+
+// SendToUser forwards an admin message for one user to the bot and reports whether
+// it was delivered. A missing bot maps to Unavailable (parity with an unreachable
+// connector); a deadline before the Ack reports delivered=false.
+func (r *RelayServer) SendToUser(ctx context.Context, req *telegramv1.SendToUserRequest) (*telegramv1.SendResponse, error) {
+	delivered, err := r.await(ctx, SendToUserCommand(req.GetExternalId(), req.GetText()))
+	if err != nil {
+		return nil, err
+	}
+	return &telegramv1.SendResponse{Delivered: delivered}, nil
+}
+
+// SendToGameChannel forwards an admin message for the game channel to the bot and
+// reports whether it was delivered.
+func (r *RelayServer) SendToGameChannel(ctx context.Context, req *telegramv1.SendToGameChannelRequest) (*telegramv1.SendResponse, error) {
+	delivered, err := r.await(ctx, SendToGameChannelCommand(req.GetText()))
+	if err != nil {
+		return nil, err
+	}
+	return &telegramv1.SendResponse{Delivered: delivered}, nil
+}
+
+// await sends cmd with the relay deadline and translates the Hub outcome into the
+// gRPC contract the backend client expects.
+func (r *RelayServer) await(ctx context.Context, cmd *botlinkv1.Command) (bool, error) {
+	cctx, cancel := context.WithTimeout(ctx, r.deadline)
+	defer cancel()
+	delivered, err := r.hub.SendAwait(cctx, cmd)
+	switch {
+	case errors.Is(err, ErrNoBot):
+		return false, status.Error(codes.Unavailable, "no telegram bot connected")
+	case err != nil:
+		return false, status.Error(codes.Internal, err.Error())
+	default:
+		return delivered, nil
+	}
+}