Stage 6: gateway edge (Connect/FlatBuffers over h2c, platform/email/guest auth, sessions, rate-limit, admin passthrough, live push bridge)

New public ingress and the first network edge. Framework + a vertical slice of
operations end-to-end; remaining ops reuse the same transcode pattern in Stage 7.

Contracts (new module scrabble/pkg):
- push.proto (backend->gateway gRPC server-stream) + scrabble.fbs (FlatBuffers
  edge payloads), committed generated Go; buf/flatc Makefiles (dev-time codegen).

Backend:
- REST handlers on the /api/v1 groups: internal session endpoints
  (telegram/guest/email login -> mint, resolve, revoke) and the user slice
  (profile, submit_play, state, lobby enqueue/poll, chat).
- internal/notify in-process Publisher hub + internal/pushgrpc gRPC server
  (BACKEND_GRPC_ADDR) streaming your_turn/opponent_moved/chat/nudge/match_found;
  emission in game.commit, social, matchmaker.
- migration 00005 accounts.is_guest; guests are durable rows excluded from stats;
  ProvisionGuest; email-as-login (RequestLoginCode/LoginWithCode).

Gateway (new module scrabble/gateway):
- Connect Gateway service over h2c (Execute + Subscribe), FlatBuffers<->JSON
  transcode registry, Telegram initData HMAC validator (seam), session cache,
  token-bucket rate limiter (3 classes), push fan-out hub, backend REST + push
  gRPC client, admin Basic-Auth reverse proxy.

go.work: use ./pkg, ./gateway + replace scrabble/pkg. CI: gateway/**, pkg/**
path filters; unit build/vet/test span all three modules. Docs (PLAN,
ARCHITECTURE, FUNCTIONAL+ru, TESTING, READMEs) updated; gateway/pkg unit tests +
guest/email-login integration tests.

pkg/fbs/scrabblefb/SubmitPlayRequest.go

@@ -0,0 +1,97 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type SubmitPlayRequest struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsSubmitPlayRequest(buf []byte, offset flatbuffers.UOffsetT) *SubmitPlayRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &SubmitPlayRequest{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishSubmitPlayRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsSubmitPlayRequest(buf []byte, offset flatbuffers.UOffsetT) *SubmitPlayRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &SubmitPlayRequest{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedSubmitPlayRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *SubmitPlayRequest) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *SubmitPlayRequest) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *SubmitPlayRequest) GameId() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *SubmitPlayRequest) Dir() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *SubmitPlayRequest) Tiles(obj *TileRecord, j int) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		x := rcv._tab.Vector(o)
+		x += flatbuffers.UOffsetT(j) * 4
+		x = rcv._tab.Indirect(x)
+		obj.Init(rcv._tab.Bytes, x)
+		return true
+	}
+	return false
+}
+
+func (rcv *SubmitPlayRequest) TilesLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
+func SubmitPlayRequestStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func SubmitPlayRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
+}
+func SubmitPlayRequestAddDir(builder *flatbuffers.Builder, dir flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(dir), 0)
+}
+func SubmitPlayRequestAddTiles(builder *flatbuffers.Builder, tiles flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(tiles), 0)
+}
+func SubmitPlayRequestStartTilesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
+func SubmitPlayRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}