Phase 28 (Step 2): FBS schemas + message-type constants for mail

Adds the wire schema for the eight `user.games.mail.*` ConnectRPC
commands together with the shared payload types (`MailMessage`,
`MailRecipientState`, `MailBroadcastReceipt`). Send-request tables
carry the optional `recipient_race_name` introduced in Step 1.

Drops:

- `pkg/schema/fbs/diplomail.fbs` — schema sources;
- `pkg/schema/fbs/diplomail/*.go` — generated Go bindings (flatc
  `--go --go-module-name galaxy/schema/fbs`);
- `pkg/model/diplomail/diplomail.go` — message-type catalog used by
  the gateway router;
- `ui/frontend/src/proto/galaxy/fbs/diplomail/*.ts` — generated TS
  bindings consumed by the upcoming UI client wrapper;
- `ui/Makefile` `FBS_INPUTS` extended to pick the new schema up on
  the next `make -C ui fbs-ts` run.

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

pkg/model/diplomail/diplomail.go

@@ -0,0 +1,56 @@
+// Package diplomail defines the public typed command identifiers
+// exposed at the authenticated Gateway -> Diplomatic Mail boundary.
+//
+// The gateway routes each `user.games.mail.*` ExecuteCommand into the
+// matching `/api/v1/user/games/{game_id}/mail/*` REST endpoint on the
+// backend; the wire envelopes and payload tables live in
+// `pkg/schema/fbs/diplomail.fbs`.
+package diplomail
+
+const (
+	// MessageTypeUserGamesMailInbox is the authenticated gateway
+	// message type used to read the caller's diplomatic-mail inbox
+	// for one game. Backend filters out rows whose `available_at` is
+	// still nil (translation in flight).
+	MessageTypeUserGamesMailInbox = "user.games.mail.inbox"
+
+	// MessageTypeUserGamesMailSent is the authenticated gateway
+	// message type used to read the caller's outgoing personal
+	// messages for one game. Admin and system rows are not included.
+	MessageTypeUserGamesMailSent = "user.games.mail.sent"
+
+	// MessageTypeUserGamesMailMessageGet is the authenticated
+	// gateway message type used to read a single message detail
+	// addressed to the caller. The response carries the translation
+	// rendering when one is cached for the caller's preferred
+	// language.
+	MessageTypeUserGamesMailMessageGet = "user.games.mail.message.get"
+
+	// MessageTypeUserGamesMailSend is the authenticated gateway
+	// message type used to send a single-recipient personal message.
+	// Exactly one of `recipient_user_id` and `recipient_race_name`
+	// must be supplied; the backend resolves the race-name shortcut
+	// through `Memberships.ListMembers(gameID, "active")`.
+	MessageTypeUserGamesMailSend = "user.games.mail.send"
+
+	// MessageTypeUserGamesMailBroadcast is the authenticated gateway
+	// message type used by paid-tier callers to broadcast a personal
+	// message to every other active member of the game.
+	MessageTypeUserGamesMailBroadcast = "user.games.mail.broadcast"
+
+	// MessageTypeUserGamesMailAdmin is the authenticated gateway
+	// message type used by the game owner to compose an admin-kind
+	// notification. The wire shape is target-discriminated: `user`
+	// addresses a single recipient (by id or race name); `all`
+	// broadcasts to every member matching the requested scope.
+	MessageTypeUserGamesMailAdmin = "user.games.mail.admin"
+
+	// MessageTypeUserGamesMailRead is the authenticated gateway
+	// message type used to mark a single message as read. Idempotent.
+	MessageTypeUserGamesMailRead = "user.games.mail.read"
+
+	// MessageTypeUserGamesMailDelete is the authenticated gateway
+	// message type used to soft-delete a single message. The
+	// recipient row must already be marked read.
+	MessageTypeUserGamesMailDelete = "user.games.mail.delete"
+)