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.

go.work

@@ -2,8 +2,19 @@ go 1.26.3
 
 use ./backend
 
+use (
+	./gateway
+	./pkg
+)
+
 // The scrabble-solver engine is consumed in-process as a library. Its module
 // path is the bare "scrabble-solver" (not a URL), so it cannot be fetched as a
 // versioned dependency via VCS; the workspace points it at the sibling checkout.
 // CI clones that sibling next to this repository before building.
 replace scrabble-solver => ../scrabble-solver
+
+// scrabble/pkg holds the shared wire contracts (push proto + FlatBuffers edge
+// payloads) imported by both backend and gateway. Its module path has no dot, so
+// like scrabble-solver it cannot be fetched as a versioned dependency; the
+// replace points the v0.0.0 require at the in-repo module directory.
+replace scrabble/pkg v0.0.0 => ./pkg