feat(export): server-rendered artifacts behind one signed download URL (#160)

The finished-game export (GCG + a new PNG of the final position) is one
signed, short-lived relative URL (game.export_url; HMAC-SHA256, 10-min
TTL, BACKEND_EXPORT_SIGN_KEY) resolved against the client's own origin
and delivered by the best affordance each platform has (five on-device
review rounds):

- TG Android/desktop: native showPopup chooser -> native downloadFile
  dialog (bridge-only chain, activation-safe).
- TG iOS: app-modal chooser -> OS share sheet with the fetched file
  (a popup callback cannot supply the activation the sheet needs).
- VK iOS: VKWebAppDownloadFile for both formats.
- VK Android: the PNG opens in VK's native image viewer, the GCG copies
  to the clipboard (the VK Android downloader hangs on any download,
  Content-Length/Range notwithstanding).
- VK desktop iframe / desktop browsers: plain anchor downloads.
- Mobile browsers: the OS share sheet (fetch-then-share).
- Legacy TG (< Bot API 8.0): app modal + GCG clipboard, no image option.

The PNG is rasterized on demand by the new internal `renderer` sidecar
(node:22-slim + skia-canvas + baked Liberation/Noto Color Emoji fonts)
executing the SAME ui/src/lib/gameimage.ts the ui project unit-tests;
the backend rebuilds the render payload from the journal +
engine.AlphabetTable, and the device date locale, IANA time zone and
localized non-play labels ride the signed URL. Nothing is stored — the
artifact re-derives from the immutable journal on each GET. The gateway
forwards /dl/* (caddy @gateway matcher extended) behind the per-IP
public rate limiter and serves bytes via http.ServeContent.

Deploy: renderer service in compose + prod overlay + rolling order +
prod push list; TEST_/PROD_EXPORT_SIGN_KEY secrets; the sidecar smoke
runs in the ui CI job. Docs: ARCHITECTURE, FUNCTIONAL(+_ru), UI_DESIGN,
TESTING, deploy/README, renderer/README.

gateway/internal/transcode/transcode_social.go

@@ -31,6 +31,7 @@ const (
 	MsgProfileUpdate     = "profile.update"
 	MsgStatsGet          = "stats.get"
 	MsgGameGCG           = "game.gcg"
+	MsgGameExportURL     = "game.export_url"
 )
 
 // registerSocialOps adds the social, account and history operations to the
@@ -56,6 +57,7 @@ func registerSocialOps(r *Registry, backend *backendclient.Client) {
 	r.ops[MsgProfileUpdate] = Op{Handler: profileUpdateHandler(backend), Auth: true}
 	r.ops[MsgStatsGet] = Op{Handler: statsHandler(backend), Auth: true}
 	r.ops[MsgGameGCG] = Op{Handler: gcgHandler(backend), Auth: true}
+	r.ops[MsgGameExportURL] = Op{Handler: exportURLHandler(backend), Auth: true}
 }
 
 // --- friends ---
@@ -290,6 +292,21 @@ func gcgHandler(backend *backendclient.Client) Handler {
 	}
 }
 
+func exportURLHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsExportUrlRequest(req.Payload, 0)
+		labels := make([]string, 0, in.ActionLabelsLength())
+		for i := range in.ActionLabelsLength() {
+			labels = append(labels, string(in.ActionLabels(i)))
+		}
+		res, err := backend.ExportURL(ctx, req.UserID, string(in.GameId()), string(in.Kind()), string(in.DateLocale()), string(in.TimeZone()), labels)
+		if err != nil {
+			return nil, err
+		}
+		return encodeExportURL(res), nil
+	}
+}
+
 // decodeInviteeIDs reads the invitee id vector from a CreateInvitationRequest.
 func decodeInviteeIDs(in *fb.CreateInvitationRequest) []string {
 	n := in.InviteeIdsLength()