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/server/handlers.go

@@ -87,12 +87,17 @@ func (s *Server) registerRoutes() {
 		u.POST("/games/:id/complaint", s.handleComplaint)
 		u.GET("/games/:id/history", s.handleHistory)
 		u.GET("/games/:id/gcg", s.handleExportGCG)
+		u.GET("/games/:id/export-url", s.handleExportURL)
 		u.GET("/games/:id/draft", s.handleGetDraft)
 		u.PUT("/games/:id/draft", s.handleSaveDraft)
 		u.POST("/games/:id/hide", s.handleHideGame)
 		// Raw dictionary download for the client-side local move preview, keyed by
 		// the game's pinned (variant, version); immutable, so cached hard.
 		u.GET("/dict/:variant/:version", s.handleDictBytes)
+		// The signed finished-game export download — the only public data route:
+		// the URL's HMAC + expiry are the whole grant (export.go), because the
+		// platforms' native download calls carry no credentials.
+		s.public.GET("/dl/:id/:kind", s.handleExportDownload)
 	}
 	if s.feedback != nil {
 		u.POST("/feedback", s.handleFeedbackSubmit)