Phase 28 (Step 3): gateway translators for user.games.mail.*

Adds the gateway-side translation layer that maps the eight new
ConnectRPC mail commands onto backend's
`/api/v1/user/games/{game_id}/mail/*` REST endpoints.

- `gateway/internal/backendclient/mail_commands.go` defines
  `ExecuteMailCommand` and one helper per command (inbox, sent,
  message.get, send, broadcast, admin, read, delete). Each helper
  decodes the FlatBuffers request envelope, issues the REST call
  via the existing `*RESTClient.do`, decodes the JSON body, and
  re-encodes a typed FlatBuffers response. Recipient identifiers
  travel through unchanged so the new `recipient_race_name`
  shortcut introduced in Step 1 reaches backend untouched.
- `routes.go` exposes a `MailRoutes` constructor and a matching
  `mailCommandClient` implementing `downstream.Client`.
- `cmd/gateway/main.go` registers the new routes alongside the
  existing user / lobby / game-engine routes.
- `mail_commands_test.go` covers the inbox, send-by-race-name, and
  read-state paths end-to-end against an `httptest.Server`,
  asserting request shapes (path, body, X-User-ID) and the
  decoded FlatBuffers response.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gateway/cmd/gateway/main.go

@@ -186,7 +186,8 @@ func newAuthenticatedGRPCDependencies(ctx context.Context, cfg config.Config, lo
 	userRoutes := backendclient.UserRoutes(backend.REST())
 	lobbyRoutes := backendclient.LobbyRoutes(backend.REST())
 	gameRoutes := backendclient.GameRoutes(backend.REST())
-	allRoutes := make(map[string]downstream.Client, len(userRoutes)+len(lobbyRoutes)+len(gameRoutes))
+	mailRoutes := backendclient.MailRoutes(backend.REST())
+	allRoutes := make(map[string]downstream.Client, len(userRoutes)+len(lobbyRoutes)+len(gameRoutes)+len(mailRoutes))
 	for k, v := range userRoutes {
 		allRoutes[k] = v
 	}
@@ -196,6 +197,9 @@ func newAuthenticatedGRPCDependencies(ctx context.Context, cfg config.Config, lo
 	for k, v := range gameRoutes {
 		allRoutes[k] = v
 	}
+	for k, v := range mailRoutes {
+		allRoutes[k] = v
+	}
 
 	cleanup := func() error {
 		return closeRedisClient()