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.

.gitea/workflows/prod-deploy.yaml

@@ -59,10 +59,10 @@ jobs:
         working-directory: deploy
         run: |
           export TAG="${{ steps.ver.outputs.tag }}" APP_VERSION="${{ steps.ver.outputs.tag }}" SCRABBLE_CONFIG_DIR=.
-          # The four main-stack images via compose (reuses the build args, incl. VERSION);
+          # The main-stack images via compose (reuses the build args, incl. VERSION);
           # the bot separately, since it is profiled out of the prod compose.
           docker compose -f docker-compose.yml -f docker-compose.prod.yml build
-          docker compose -f docker-compose.yml -f docker-compose.prod.yml push backend gateway landing validator
+          docker compose -f docker-compose.yml -f docker-compose.prod.yml push backend gateway landing validator renderer
           docker build -f ../platform/telegram/Dockerfile --target bot --build-arg VERSION="$TAG" -t "$REGISTRY/scrabble-telegram-bot:$TAG" ..
           docker push "$REGISTRY/scrabble-telegram-bot:$TAG"
 
@@ -84,6 +84,8 @@ jobs:
       GRAFANA_ADMIN_PASSWORD: ${{ secrets.PROD_GRAFANA_ADMIN_PASSWORD }}
       TELEGRAM_BOT_TOKEN: ${{ secrets.PROD_TELEGRAM_BOT_TOKEN }}
       GATEWAY_VK_APP_SECRET: ${{ secrets.PROD_GATEWAY_VK_APP_SECRET }}
+      # Signs the finished-game export download URLs (backend BACKEND_EXPORT_SIGN_KEY).
+      EXPORT_SIGN_KEY: ${{ secrets.PROD_EXPORT_SIGN_KEY }}
       PROD_BOTLINK_CA: ${{ secrets.PROD_BOTLINK_CA }}
       PROD_BOTLINK_GATEWAY_CERT: ${{ secrets.PROD_BOTLINK_GATEWAY_CERT }}
       PROD_BOTLINK_GATEWAY_KEY: ${{ secrets.PROD_BOTLINK_GATEWAY_KEY }}
@@ -139,6 +141,7 @@ jobs:
           export TELEGRAM_BOT_TOKEN='$TELEGRAM_BOT_TOKEN'
           export TELEGRAM_MINIAPP_URL='$TELEGRAM_MINIAPP_URL'
           export GATEWAY_VK_APP_SECRET='$GATEWAY_VK_APP_SECRET'
+          export EXPORT_SIGN_KEY='$EXPORT_SIGN_KEY'
           export GATEWAY_ABUSE_BAN_ENABLED='true'
           EOF
           printf '%s\n' "$PROD_BOTLINK_CA"           > stage/certs-main/ca.crt