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.

pkg/fbs/scrabble.fbs

@@ -658,6 +658,27 @@ table GcgExport {
   content:string;
 }
 
+// ExportUrlRequest asks for a signed download URL of a finished game's export artifact.
+// kind is "png" or "gcg". For the PNG the client passes its presentation context — the
+// UI-localized non-play move labels (pass, exchange, resign, timeout, in that order),
+// the device date locale and the device IANA time zone — which ride the signed URL so
+// the server render matches what the player would have seen locally.
+table ExportUrlRequest {
+  game_id:string;
+  kind:string;
+  date_locale:string;
+  action_labels:[string];
+  time_zone:string;
+}
+
+// ExportUrl is the minted download link: a relative, signed, short-lived path the client
+// resolves against its own origin (the SPA and the gateway share it), plus the filename
+// the download will carry.
+table ExportUrl {
+  path:string;
+  filename:string;
+}
+
 // --- push event payloads ---
 
 // YourTurnEvent signals that it is now the recipient's turn. The trailing fields enrich the