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.

backend/internal/render/render.go

@@ -0,0 +1,61 @@
+// Package render is the backend's client for the internal image-render sidecar: it POSTs
+// a finished game's export payload and receives the rasterized PNG. The sidecar runs the
+// same drawing code the web client unit-tests (ui/src/lib/gameimage.ts on skia-canvas);
+// authentication and the signed public URL live in the server layer — this client only
+// carries bytes.
+package render
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+// maxPNG caps a render response; a scale-2 export of the largest plausible game is well
+// under 2 MiB, so anything bigger is a sidecar malfunction, not a valid image.
+const maxPNG = 8 << 20
+
+// Client calls the render sidecar over the internal network.
+type Client struct {
+	base string
+	http *http.Client
+}
+
+// New returns a client for the sidecar at baseURL (e.g. http://renderer:8090). A render
+// of a full game takes well under a second; the timeout leaves room for a cold start.
+func New(baseURL string) *Client {
+	return &Client{base: baseURL, http: &http.Client{Timeout: 15 * time.Second}}
+}
+
+// Render posts payload (the JSON request shape the sidecar consumes) and returns the PNG.
+func (c *Client) Render(ctx context.Context, payload any) ([]byte, error) {
+	body, err := json.Marshal(payload)
+	if err != nil {
+		return nil, fmt.Errorf("render: marshal payload: %w", err)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.base+"/render", bytes.NewReader(body))
+	if err != nil {
+		return nil, fmt.Errorf("render: build request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("render: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("render: sidecar status %d", resp.StatusCode)
+	}
+	png, err := io.ReadAll(io.LimitReader(resp.Body, maxPNG+1))
+	if err != nil {
+		return nil, fmt.Errorf("render: read response: %w", err)
+	}
+	if len(png) > maxPNG {
+		return nil, fmt.Errorf("render: response exceeds %d bytes", maxPNG)
+	}
+	return png, nil
+}