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  `forbidden` when the matching member is no longer active;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/README.md

@@ -26,7 +26,10 @@ Three Postgres tables in the `backend` schema:
 
 - `diplomail_messages` — one row per send (personal, admin, or
   system). Captures `game_name` and IP at insert time so audit
-  rendering survives renames and purges.
+  rendering survives renames and purges. The `sender_race_name`
+  column snapshots the sender's race in the game at send time when
+  the sender is a player with an active membership; the in-game UI
+  keys per-race thread grouping on this column.
 - `diplomail_recipients` — one row per (message, recipient). Holds
   per-user `read_at`, `deleted_at`, `delivered_at`, `notified_at`
   state. Snapshot fields (`recipient_user_name`,
@@ -72,6 +75,24 @@ mail to every active member; `Service.changeMembershipStatus` /
   `detector.LanguageDetector` (default: `whatlanggo`, body-only,
   ≥ 25 runes; shorter bodies stay `und`).
 
+## Recipient selection
+
+`POST /messages` and `POST /admin` (when `target="user"`) accept the
+recipient identifier in one of two shapes:
+
+- `recipient_user_id` (uuid) — explicit user lookup; the recipient
+  may be any active member of the game.
+- `recipient_race_name` (string) — resolves to the active member
+  with this race name in the game. Race names are unique by lobby
+  invariant; lobby-removed and blocked members cannot be reached
+  through the race-name shortcut (they no longer appear in the
+  active scope). Exactly one of the two fields must be supplied;
+  supplying both, or neither, returns `invalid_request`.
+
+The race-name path lets the in-game UI compose mail directly off
+the engine's `report.races[]` view without an extra membership
+round-trip.
+
 ## Translation
 
 Stage D adds a lazy translation cache. When a recipient reads a