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.

pkg/fbs/scrabble.fbs

@@ -658,6 +658,27 @@ table GcgExport {
   content:string;
 }
 
+// ExportUrlRequest asks for a signed download URL of a finished game's export artifact.
+// kind is "png" or "gcg". For the PNG the client passes its presentation context — the
+// UI-localized non-play move labels (pass, exchange, resign, timeout, in that order),
+// the device date locale and the device IANA time zone — which ride the signed URL so
+// the server render matches what the player would have seen locally.
+table ExportUrlRequest {
+  game_id:string;
+  kind:string;
+  date_locale:string;
+  action_labels:[string];
+  time_zone:string;
+}
+
+// ExportUrl is the minted download link: a relative, signed, short-lived path the client
+// resolves against its own origin (the SPA and the gateway share it), plus the filename
+// the download will carry.
+table ExportUrl {
+  path:string;
+  filename:string;
+}
+
 // --- push event payloads ---
 
 // YourTurnEvent signals that it is now the recipient's turn. The trailing fields enrich the

pkg/fbs/scrabblefb/ExportUrl.go

@@ -0,0 +1,71 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type ExportUrl struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsExportUrl(buf []byte, offset flatbuffers.UOffsetT) *ExportUrl {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &ExportUrl{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishExportUrlBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsExportUrl(buf []byte, offset flatbuffers.UOffsetT) *ExportUrl {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &ExportUrl{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedExportUrlBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *ExportUrl) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *ExportUrl) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *ExportUrl) Path() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExportUrl) Filename() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func ExportUrlStart(builder *flatbuffers.Builder) {
+	builder.StartObject(2)
+}
+func ExportUrlAddPath(builder *flatbuffers.Builder, path flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(path), 0)
+}
+func ExportUrlAddFilename(builder *flatbuffers.Builder, filename flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(filename), 0)
+}
+func ExportUrlEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}

pkg/fbs/scrabblefb/ExportUrlRequest.go

@@ -0,0 +1,116 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type ExportUrlRequest struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsExportUrlRequest(buf []byte, offset flatbuffers.UOffsetT) *ExportUrlRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &ExportUrlRequest{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishExportUrlRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsExportUrlRequest(buf []byte, offset flatbuffers.UOffsetT) *ExportUrlRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &ExportUrlRequest{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedExportUrlRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *ExportUrlRequest) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *ExportUrlRequest) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *ExportUrlRequest) GameId() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExportUrlRequest) Kind() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExportUrlRequest) DateLocale() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExportUrlRequest) ActionLabels(j int) []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+	}
+	return nil
+}
+
+func (rcv *ExportUrlRequest) ActionLabelsLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
+func (rcv *ExportUrlRequest) TimeZone() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(12))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func ExportUrlRequestStart(builder *flatbuffers.Builder) {
+	builder.StartObject(5)
+}
+func ExportUrlRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
+}
+func ExportUrlRequestAddKind(builder *flatbuffers.Builder, kind flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(kind), 0)
+}
+func ExportUrlRequestAddDateLocale(builder *flatbuffers.Builder, dateLocale flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(dateLocale), 0)
+}
+func ExportUrlRequestAddActionLabels(builder *flatbuffers.Builder, actionLabels flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(3, flatbuffers.UOffsetT(actionLabels), 0)
+}
+func ExportUrlRequestStartActionLabelsVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
+func ExportUrlRequestAddTimeZone(builder *flatbuffers.Builder, timeZone flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(4, flatbuffers.UOffsetT(timeZone), 0)
+}
+func ExportUrlRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}