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.

renderer/src/render.mjs

@@ -0,0 +1,24 @@
+// The render core, separated from the HTTP wiring so the node test can call it directly.
+import { Canvas } from 'skia-canvas';
+import { drawGameImage, setAlphabet } from '../dist/gameimage.mjs';
+
+/**
+ * renderRequest rasterizes one export request — the JSON shape the backend sends:
+ * { game, moves, alphabet, labels, dateLocale, hostname, scale? } — into a PNG buffer.
+ * game/moves are the ui-model shapes (GameView / MoveRecord[]) the shared drawGameImage
+ * consumes; alphabet is the per-variant (index, letter, value) table it scores tiles with.
+ */
+export async function renderRequest(payload) {
+  const { game, moves, alphabet, labels, dateLocale, timeZone, hostname, scale } = payload;
+  if (!game || !Array.isArray(moves)) throw Object.assign(new Error('bad payload'), { status: 400 });
+  setAlphabet(game.variant, alphabet ?? []);
+  const canvas = new Canvas(1, 1);
+  drawGameImage(canvas, game, moves, {
+    actionLabel: (a) => (labels && labels[a]) || a,
+    hostname: hostname || '',
+    dateLocale: dateLocale || undefined,
+    timeZone: timeZone || undefined,
+    scale: scale || 2,
+  });
+  return canvas.toBuffer('png');
+}