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  `forbidden` when the matching member is no longer active;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/openapi.yaml

@@ -4068,11 +4068,22 @@ components:
     UserMailSendRequest:
       type: object
       additionalProperties: false
-      required: [recipient_user_id, body]
+      required: [body]
       properties:
         recipient_user_id:
           type: string
           format: uuid
+          description: |
+            Either `recipient_user_id` or `recipient_race_name` must
+            be supplied; supplying both is rejected as
+            `invalid_request`.
+        recipient_race_name:
+          type: string
+          description: |
+            Resolves to the active member with this race name in the
+            game. Mutually exclusive with `recipient_user_id`. The
+            server returns `forbidden` when the matching member is no
+            longer active (lobby-removed / blocked).
         subject:
           type: string
           description: |
@@ -4093,10 +4104,18 @@ components:
           type: string
           format: uuid
           description: |
-            Required when `target="user"`. Identifies the recipient
-            of the personal admin message; the recipient may be in
-            any membership status (admin notifications can reach
-            kicked players).
+            One of `recipient_user_id` and `recipient_race_name` is
+            required when `target="user"`. Identifies the recipient
+            of the personal admin message by uuid; the recipient may
+            be in any membership status (admin notifications can
+            reach kicked players when addressed by user_id).
+        recipient_race_name:
+          type: string
+          description: |
+            Optional alternative to `recipient_user_id` when
+            `target="user"`. Resolves to the active member with this
+            race name in the game; lobby-removed and blocked members
+            cannot be reached through the race-name shortcut.
         recipients:
           type: string
           enum: [active, active_and_removed, all_members]
@@ -4323,6 +4342,17 @@ components:
         sender_username:
           type: string
           nullable: true
+        sender_race_name:
+          type: string
+          nullable: true
+          description: |
+            Snapshot of the sender's race name in this game at send
+            time. Populated when `sender_kind="player"` and the
+            sender had an active membership at send time; nil for
+            admin and system messages, and for player messages sent
+            by a private-game owner who was not an active member at
+            send time. The in-game UI keys per-race threading on this
+            field.
         subject:
           type: string
         body: