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.

backend/internal/account/account.go

@@ -1,6 +1,8 @@
 // Package account owns durable internal accounts and their platform/email
 // identities. First contact from a platform auto-provisions an account bound to
-// that identity; guests are session-only and never reach this package.
+// that identity. An ephemeral guest is also a durable account row (the sessions
+// and game_players foreign keys both require one) but carries no identity and is
+// flagged is_guest, which excludes it from statistics, friends and history.
 package account
 
 import (
@@ -52,8 +54,11 @@ type Account struct {
 	HintBalance         int
 	BlockChat           bool
 	BlockFriendRequests bool
-	CreatedAt           time.Time
-	UpdatedAt           time.Time
+	// IsGuest marks an ephemeral guest account: a durable row with no identity,
+	// excluded from statistics, friends and history.
+	IsGuest   bool
+	CreatedAt time.Time
+	UpdatedAt time.Time
 }
 
 // Store is the Postgres-backed query surface for accounts and identities.
@@ -176,6 +181,30 @@ func (s *Store) create(ctx context.Context, kind, externalID string) (Account, e
 	return created, nil
 }
 
+// guestDisplayName is the display name stamped on a freshly provisioned guest.
+const guestDisplayName = "Guest"
+
+// ProvisionGuest creates a fresh ephemeral guest account: a durable row carrying
+// no identity, flagged is_guest, so it can hold a session and a game seat (both
+// foreign-key the accounts table) while being excluded from statistics, friends
+// and history. Guests are not reused — each bootstrap mints a new account.
+func (s *Store) ProvisionGuest(ctx context.Context) (Account, error) {
+	accountID, err := uuid.NewV7()
+	if err != nil {
+		return Account{}, fmt.Errorf("account: new guest id: %w", err)
+	}
+	stmt := table.Accounts.
+		INSERT(table.Accounts.AccountID, table.Accounts.DisplayName, table.Accounts.IsGuest).
+		VALUES(accountID, guestDisplayName, true).
+		RETURNING(table.Accounts.AllColumns)
+
+	var row model.Accounts
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		return Account{}, fmt.Errorf("account: provision guest: %w", err)
+	}
+	return modelToAccount(row), nil
+}
+
 // SpendHint atomically decrements the account's hint wallet by one, returning
 // true when a hint was spent and false when the balance was already empty. The
 // guarded UPDATE keeps it safe under concurrent spends across the player's games.
@@ -210,6 +239,7 @@ func modelToAccount(row model.Accounts) Account {
 		HintBalance:         int(row.HintBalance),
 		BlockChat:           row.BlockChat,
 		BlockFriendRequests: row.BlockFriendRequests,
+		IsGuest:             row.IsGuest,
 		CreatedAt:           row.CreatedAt,
 		UpdatedAt:           row.UpdatedAt,
 	}