Phase 28 (Step 1): backend support for race-name mail send

Phase 28's in-game mail UI groups personal threads by the other
party's race. To support that without an extra membership-listing
RPC, the diplomail subsystem now:

- accepts `recipient_race_name` on `POST /messages` and
  `POST /admin` (target=user) as an alternative to
  `recipient_user_id`; the service resolves it via the existing
  `Memberships.ListMembers(gameID, "active")` and rejects with
  `forbidden` when the matching member is no longer active;
- snapshots `diplomail_messages.sender_race_name` at send time for
  every player sender (admin / system rows stay NULL). The UI keys
  per-race threading on this column.

Schema, openapi, README, and a focused e2e test for the new path
(happy path + dual / missing / unknown / kicked errors) land in
this commit; the gateway + UI legs follow in subsequent commits on
this branch.

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

backend/internal/diplomail/store.go

@@ -31,7 +31,7 @@ func messageColumns() postgres.ColumnList {
 	m := table.DiplomailMessages
 	return postgres.ColumnList{
 		m.MessageID, m.GameID, m.GameName, m.Kind, m.SenderKind,
-		m.SenderUserID, m.SenderUsername, m.SenderIP,
+		m.SenderUserID, m.SenderUsername, m.SenderRaceName, m.SenderIP,
 		m.Subject, m.Body, m.BodyLang, m.BroadcastScope, m.CreatedAt,
 	}
 }
@@ -59,6 +59,7 @@ type MessageInsert struct {
 	SenderKind     string
 	SenderUserID   *uuid.UUID
 	SenderUsername *string
+	SenderRaceName *string
 	SenderIP       string
 	Subject        string
 	Body           string
@@ -101,7 +102,7 @@ func (s *Store) InsertMessageWithRecipients(ctx context.Context, msg MessageInse
 	m := table.DiplomailMessages
 	msgStmt := m.INSERT(
 		m.MessageID, m.GameID, m.GameName, m.Kind, m.SenderKind,
-		m.SenderUserID, m.SenderUsername, m.SenderIP,
+		m.SenderUserID, m.SenderUsername, m.SenderRaceName, m.SenderIP,
 		m.Subject, m.Body, m.BodyLang, m.BroadcastScope,
 	).VALUES(
 		msg.MessageID,
@@ -111,6 +112,7 @@ func (s *Store) InsertMessageWithRecipients(ctx context.Context, msg MessageInse
 		msg.SenderKind,
 		uuidPtrArg(msg.SenderUserID),
 		stringPtrArg(msg.SenderUsername),
+		stringPtrArg(msg.SenderRaceName),
 		msg.SenderIP,
 		msg.Subject,
 		msg.Body,
@@ -737,6 +739,10 @@ func messageFromModel(row model.DiplomailMessages) Message {
 		name := *row.SenderUsername
 		out.SenderUsername = &name
 	}
+	if row.SenderRaceName != nil {
+		name := *row.SenderRaceName
+		out.SenderRaceName = &name
+	}
 	return out
 }