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/.env.example

@@ -21,6 +21,11 @@ DICT_VERSION=v1.3.0
 # --- Logging ----------------------------------------------------------------
 LOG_LEVEL=info
 
+# --- Finished-game export ----------------------------------------------------
+# HMAC key signing the public export download URLs (/dl/*). Required; generate a
+# real contour value with `openssl rand -base64 32` (Gitea TEST_/PROD_EXPORT_SIGN_KEY).
+EXPORT_SIGN_KEY=dev-export-sign-key
+
 # --- Edge / caddy -----------------------------------------------------------
 # Test: ":80" (the host caddy terminates TLS and forwards to scrabble:80 on the
 # external `edge` network). Prod: a domain so caddy does its own ACME.