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.

backend/internal/config/config.go

@@ -51,6 +51,12 @@ type Config struct {
 	// GuestRetention is the account age past which an unused guest (no game seat)
 	// is eligible for deletion by the reaper.
 	GuestRetention time.Duration
+	// ExportSignKey signs the finished-game export download URLs. Empty leaves
+	// the export-URL endpoints disabled (503 on mint, 404 on download).
+	ExportSignKey string
+	// RendererURL is the base URL of the internal image-render sidecar (e.g.
+	// http://renderer:8090). Empty disables the PNG export artifact.
+	RendererURL string
 }
 
 // Defaults applied when the corresponding environment variable is unset.
@@ -151,6 +157,8 @@ func Load() (Config, error) {
 		ConnectorAddr:     os.Getenv("BACKEND_CONNECTOR_ADDR"),
 		GuestReapInterval: guestReapInterval,
 		GuestRetention:    guestRetention,
+		ExportSignKey:     os.Getenv("BACKEND_EXPORT_SIGN_KEY"),
+		RendererURL:       os.Getenv("BACKEND_RENDERER_URL"),
 	}
 	if err := c.validate(); err != nil {
 		return Config{}, err