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.

deploy/docker-compose.yml

@@ -61,6 +61,36 @@ services:
           memory: 512M
     networks: [internal]
 
+  # The finished-game image-render sidecar: internal-only, called by the backend for the
+  # PNG export artifact. It runs the same ui/src/lib/gameimage.ts the web client
+  # unit-tests, on skia-canvas (renderer/README.md). node:22-slim carries a shell, so —
+  # uniquely among the built services — it has a real container healthcheck the backend's
+  # depends_on gates on.
+  renderer:
+    container_name: scrabble-renderer
+    image: scrabble-renderer:latest
+    build:
+      context: ..
+      dockerfile: renderer/Dockerfile
+      args:
+        VERSION: ${APP_VERSION:-dev}
+    restart: unless-stopped
+    logging: *default-logging
+    environment:
+      RENDERER_PORT: "8090"
+    healthcheck:
+      test: ["CMD", "node", "-e", "fetch('http://127.0.0.1:8090/healthz').then((r)=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"]
+      interval: 10s
+      timeout: 3s
+      retries: 12
+    # A render peaks well under 100 MiB (a 2-MP canvas + skia); idle sits near 60 MiB.
+    deploy:
+      resources:
+        limits:
+          cpus: "1.0"
+          memory: 192M
+    networks: [internal]
+
   backend:
     container_name: scrabble-backend
     image: scrabble-backend:latest
@@ -80,9 +110,15 @@ services:
     depends_on:
       postgres:
         condition: service_healthy
+      renderer:
+        condition: service_healthy
     environment:
       # search_path=backend matches the migrations (00001 creates the schema).
       BACKEND_POSTGRES_DSN: postgres://${POSTGRES_USER:-scrabble}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB:-scrabble}?sslmode=disable&search_path=backend
+      # The finished-game export: the render sidecar address + the HMAC key signing the
+      # public download URLs (deploy env: TEST_/PROD_EXPORT_SIGN_KEY).
+      BACKEND_RENDERER_URL: http://renderer:8090
+      BACKEND_EXPORT_SIGN_KEY: ${EXPORT_SIGN_KEY:?set EXPORT_SIGN_KEY — the export download URL signing secret}
       # The pool caps at 25 conns (~28 backends) around 500 players; 40 gives headroom
       # for bursts. Postgres (2 cores / 512 MiB) handles it.
       BACKEND_POSTGRES_MAX_OPEN_CONNS: "40"