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

The finished-game export now works identically on every platform: the
client mints a signed relative URL (game.export_url) carrying its date
locale, IANA time zone and localized non-play labels, resolves it
against its own origin and hands it to the platform's native download —
Telegram downloadFile, VKWebAppDownloadFile, or a plain browser anchor
(also the desktop VK iframe). Both artifacts ride the route: the .gcg
text (no more clipboard mode, except the legacy pre-8.0 Telegram
fallback) and the PNG of the final position.

The PNG is rasterized by a new internal 'renderer' sidecar (node:22-slim
+ skia-canvas + baked Liberation/Noto Color Emoji fonts) executing the
SAME ui/src/lib/gameimage.ts the web project unit-tests, bundled at
image build time — one renderer, no drift; the browser no longer draws
or delivers bytes itself. The backend rebuilds the render payload from
the journal + engine.AlphabetTable, verifies the HMAC (10-minute TTL,
BACKEND_EXPORT_SIGN_KEY, constant-time, uniform 404s) on its public
group, and streams the artifact as a named attachment; the gateway
forwards /dl/* behind the per-IP public rate limiter (caddy @gateway
matcher extended — the landing catch-all trap).

Deploy: renderer service (compose + prod overlay + roll before backend
+ prod push list), EXPORT_SIGN_KEY env (TEST_/PROD_ secrets), CI runs
the sidecar smoke in the ui job. Docs: ARCHITECTURE export-delivery
section, FUNCTIONAL(+_ru), UI_DESIGN, TESTING, deploy/README.

backend/internal/server/server.go

@@ -29,6 +29,7 @@ import (
 	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/notify"
 	"scrabble/backend/internal/ratewatch"
+	"scrabble/backend/internal/render"
 	"scrabble/backend/internal/session"
 	"scrabble/backend/internal/social"
 	"scrabble/backend/internal/telemetry"
@@ -96,6 +97,12 @@ type Deps struct {
 	// signal the banner/hint/role console actions emit. A nil Notifier discards
 	// them (notify.Nop).
 	Notifier notify.Publisher
+	// ExportSignKey signs the finished-game export download URLs (export.go). An
+	// empty key leaves the export-URL endpoints answering 503/404.
+	ExportSignKey string
+	// Renderer is the image-render sidecar client for the PNG export artifact. A
+	// nil Renderer makes the PNG download answer 404 (the GCG artifact still works).
+	Renderer *render.Client
 }
 
 // Server owns the gin engine, the underlying HTTP server and the readiness
@@ -124,6 +131,8 @@ type Server struct {
 	ads         *ads.Service
 	notifier    notify.Publisher
 	console     *adminconsole.Renderer
+	exportKey   []byte
+	renderer    *render.Client
 
 	public   *gin.RouterGroup
 	user     *gin.RouterGroup
@@ -173,8 +182,12 @@ func New(addr string, deps Deps) *Server {
 		banview:       deps.BanView,
 		ads:           deps.Ads,
 		notifier:      notifier,
+		renderer:      deps.Renderer,
 		http:          &http.Server{Addr: addr, Handler: engine},
 	}
+	if deps.ExportSignKey != "" {
+		s.exportKey = []byte(deps.ExportSignKey)
+	}
 	s.registerProbes(engine)
 	s.registerAPIGroups(engine)
 	s.registerRoutes()