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>
Ilia Denisov