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  `forbidden` when the matching member is no longer active;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/server/handlers_admin_diplomail.go

@@ -60,19 +60,24 @@ func (h *AdminDiplomailHandlers) Send() gin.HandlerFunc {
 		ctx := c.Request.Context()
 		switch req.Target {
 		case "", "user":
-			recipientID, parseErr := uuid.Parse(req.RecipientUserID)
-			if parseErr != nil {
-				httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, "recipient_user_id must be a valid UUID")
-				return
+			var recipientID uuid.UUID
+			if req.RecipientUserID != "" {
+				parsed, parseErr := uuid.Parse(req.RecipientUserID)
+				if parseErr != nil {
+					httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, "recipient_user_id must be a valid UUID")
+					return
+				}
+				recipientID = parsed
 			}
 			msg, rcpt, sendErr := h.svc.SendAdminPersonal(ctx, diplomail.SendAdminPersonalInput{
-				GameID:          gameID,
-				CallerKind:      diplomail.CallerKindAdmin,
-				CallerUsername:  username,
-				RecipientUserID: recipientID,
-				Subject:         req.Subject,
-				Body:            req.Body,
-				SenderIP:        clientip.ExtractSourceIP(c),
+				GameID:            gameID,
+				CallerKind:        diplomail.CallerKindAdmin,
+				CallerUsername:    username,
+				RecipientUserID:   recipientID,
+				RecipientRaceName: req.RecipientRaceName,
+				Subject:           req.Subject,
+				Body:              req.Body,
+				SenderIP:          clientip.ExtractSourceIP(c),
 			})
 			if sendErr != nil {
 				respondDiplomailError(c, h.logger, "admin mail send personal", ctx, sendErr)