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/schema/fbs/diplomail.fbs

@@ -0,0 +1,196 @@
+// diplomail contains FlatBuffers payloads used by the authenticated
+// gateway boundary for the in-game diplomatic-mail subsystem. The
+// wire shapes here mirror the trusted internal
+// `/api/v1/user/games/{game_id}/mail/*` REST surface; gateway derives
+// the calling `user_id` from the verified session and forwards it as
+// `X-User-Id` to backend.
+
+include "common.fbs";
+
+namespace diplomail;
+
+// MailMessage stores one inbox / sent-list / message-detail row. The
+// fields mirror `UserMailMessageDetail` in `backend/openapi.yaml`
+// with the following encoding rules:
+//
+//   - `*_user_id` fields are RFC 4122 string UUIDs ("" means absent
+//     for nullable fields such as `sender_user_id`).
+//   - `*_at_ms` fields carry Unix milliseconds; `0` means the
+//     timestamp is absent (e.g. an unread message has
+//     `read_at_ms == 0`).
+//   - `translated_*`, `translation_lang`, and `translator` are set
+//     when the backend served a cached rendering into the caller's
+//     preferred language; empty otherwise.
+//   - `sender_race_name` is the snapshot of the sender's race name
+//     in this game at send time. Present for `sender_kind="player"`
+//     messages when the sender had an active membership; absent for
+//     admin and system messages. The in-game UI keys per-race
+//     threading on this field.
+table MailMessage {
+  message_id:string;
+  game_id:string;
+  game_name:string;
+  kind:string;
+  sender_kind:string;
+  sender_user_id:string;
+  sender_username:string;
+  sender_race_name:string;
+  subject:string;
+  body:string;
+  body_lang:string;
+  broadcast_scope:string;
+  created_at_ms:int64;
+  recipient_user_id:string;
+  recipient_user_name:string;
+  recipient_race_name:string;
+  read_at_ms:int64;
+  deleted_at_ms:int64;
+  translated_subject:string;
+  translated_body:string;
+  translation_lang:string;
+  translator:string;
+}
+
+// MailRecipientState mirrors the `UserMailRecipientState` payload
+// returned from mark-read and soft-delete endpoints. Same timestamp
+// conventions as `MailMessage`.
+table MailRecipientState {
+  message_id:string;
+  read_at_ms:int64;
+  deleted_at_ms:int64;
+}
+
+// MailBroadcastReceipt mirrors `UserMailBroadcastReceipt`. Returned
+// from broadcast sends (paid-tier and admin); `recipient_count` is
+// the number of recipient rows the server materialised.
+table MailBroadcastReceipt {
+  message_id:string;
+  game_id:string;
+  game_name:string;
+  kind:string;
+  sender_kind:string;
+  subject:string;
+  body:string;
+  body_lang:string;
+  broadcast_scope:string;
+  created_at_ms:int64;
+  recipient_count:int32;
+}
+
+// InboxRequest stores the read-side request for the caller's inbox
+// in `game_id`. Backend filters to messages with `available_at` set
+// (translation completed when the recipient's preferred language
+// differs from the body language).
+table InboxRequest {
+  game_id:common.UUID (required);
+}
+
+// InboxResponse stores the resulting inbox list, newest first.
+// `items` is empty when the caller has no available messages in
+// this game.
+table InboxResponse {
+  items:[MailMessage];
+}
+
+// SentRequest stores the read-side request for the caller's sent
+// personal messages in `game_id`. Admin / system rows are not
+// included.
+table SentRequest {
+  game_id:common.UUID (required);
+}
+
+// SentResponse stores the caller's outgoing personal-message list.
+// Each `MailMessage` carries the original recipient snapshot.
+table SentResponse {
+  items:[MailMessage];
+}
+
+// MessageGetRequest stores the read-side request for a single
+// message detail. The caller must be a recipient of the message.
+table MessageGetRequest {
+  game_id:common.UUID (required);
+  message_id:common.UUID (required);
+}
+
+// MessageGetResponse stores the fully decorated message detail
+// including any cached translation into the caller's preferred
+// language.
+table MessageGetResponse {
+  message:MailMessage;
+}
+
+// SendRequest stores the write-side request for a single-recipient
+// personal send. Exactly one of `recipient_user_id` /
+// `recipient_race_name` must be supplied; the empty string means
+// "use the other field".
+table SendRequest {
+  game_id:common.UUID (required);
+  recipient_user_id:string;
+  recipient_race_name:string;
+  subject:string;
+  body:string;
+}
+
+// SendResponse echoes the freshly inserted message detail.
+table SendResponse {
+  message:MailMessage;
+}
+
+// BroadcastRequest stores the paid-tier player broadcast. The
+// recipient set is always "every other active member of the game".
+table BroadcastRequest {
+  game_id:common.UUID (required);
+  subject:string;
+  body:string;
+}
+
+// BroadcastResponse stores the receipt returned by the server.
+table BroadcastResponse {
+  receipt:MailBroadcastReceipt;
+}
+
+// AdminRequest stores the owner-only admin send. `target="user"`
+// requires exactly one of `recipient_user_id` / `recipient_race_name`;
+// `target="all"` accepts the optional `recipients` scope (default
+// `active`).
+table AdminRequest {
+  game_id:common.UUID (required);
+  target:string;
+  recipient_user_id:string;
+  recipient_race_name:string;
+  recipients:string;
+  subject:string;
+  body:string;
+}
+
+// AdminResponse carries the result of an admin send. When the
+// request had `target="user"`, `message` is set; when `target="all"`,
+// `receipt` is set. Callers branch on which field is present.
+table AdminResponse {
+  message:MailMessage;
+  receipt:MailBroadcastReceipt;
+}
+
+// ReadRequest stores the mark-read intent for a single message. The
+// caller must be a recipient. Idempotent.
+table ReadRequest {
+  game_id:common.UUID (required);
+  message_id:common.UUID (required);
+}
+
+// ReadResponse echoes the recipient state after the operation.
+table ReadResponse {
+  state:MailRecipientState;
+}
+
+// DeleteRequest stores the soft-delete intent for a single message.
+// The message must already be marked read (HTTP 409 otherwise).
+table DeleteRequest {
+  game_id:common.UUID (required);
+  message_id:common.UUID (required);
+}
+
+// DeleteResponse echoes the recipient state after the operation.
+table DeleteResponse {
+  state:MailRecipientState;
+}