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  `forbidden` when the matching member is no longer active;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/postgres/jet/backend/model/diplomail_messages.go

@@ -20,6 +20,7 @@ type DiplomailMessages struct {
 	SenderKind     string
 	SenderUserID   *uuid.UUID
 	SenderUsername *string
+	SenderRaceName *string
 	SenderIP       string
 	Subject        string
 	Body           string

backend/internal/postgres/jet/backend/table/diplomail_messages.go

@@ -24,6 +24,7 @@ type diplomailMessagesTable struct {
 	SenderKind     postgres.ColumnString
 	SenderUserID   postgres.ColumnString
 	SenderUsername postgres.ColumnString
+	SenderRaceName postgres.ColumnString
 	SenderIP       postgres.ColumnString
 	Subject        postgres.ColumnString
 	Body           postgres.ColumnString
@@ -78,14 +79,15 @@ func newDiplomailMessagesTableImpl(schemaName, tableName, alias string) diplomai
 		SenderKindColumn     = postgres.StringColumn("sender_kind")
 		SenderUserIDColumn   = postgres.StringColumn("sender_user_id")
 		SenderUsernameColumn = postgres.StringColumn("sender_username")
+		SenderRaceNameColumn = postgres.StringColumn("sender_race_name")
 		SenderIPColumn       = postgres.StringColumn("sender_ip")
 		SubjectColumn        = postgres.StringColumn("subject")
 		BodyColumn           = postgres.StringColumn("body")
 		BodyLangColumn       = postgres.StringColumn("body_lang")
 		BroadcastScopeColumn = postgres.StringColumn("broadcast_scope")
 		CreatedAtColumn      = postgres.TimestampzColumn("created_at")
-		allColumns           = postgres.ColumnList{MessageIDColumn, GameIDColumn, GameNameColumn, KindColumn, SenderKindColumn, SenderUserIDColumn, SenderUsernameColumn, SenderIPColumn, SubjectColumn, BodyColumn, BodyLangColumn, BroadcastScopeColumn, CreatedAtColumn}
-		mutableColumns       = postgres.ColumnList{GameIDColumn, GameNameColumn, KindColumn, SenderKindColumn, SenderUserIDColumn, SenderUsernameColumn, SenderIPColumn, SubjectColumn, BodyColumn, BodyLangColumn, BroadcastScopeColumn, CreatedAtColumn}
+		allColumns           = postgres.ColumnList{MessageIDColumn, GameIDColumn, GameNameColumn, KindColumn, SenderKindColumn, SenderUserIDColumn, SenderUsernameColumn, SenderRaceNameColumn, SenderIPColumn, SubjectColumn, BodyColumn, BodyLangColumn, BroadcastScopeColumn, CreatedAtColumn}
+		mutableColumns       = postgres.ColumnList{GameIDColumn, GameNameColumn, KindColumn, SenderKindColumn, SenderUserIDColumn, SenderUsernameColumn, SenderRaceNameColumn, SenderIPColumn, SubjectColumn, BodyColumn, BodyLangColumn, BroadcastScopeColumn, CreatedAtColumn}
 		defaultColumns       = postgres.ColumnList{SenderIPColumn, SubjectColumn, BodyLangColumn, BroadcastScopeColumn, CreatedAtColumn}
 	)
 
@@ -100,6 +102,7 @@ func newDiplomailMessagesTableImpl(schemaName, tableName, alias string) diplomai
 		SenderKind:     SenderKindColumn,
 		SenderUserID:   SenderUserIDColumn,
 		SenderUsername: SenderUsernameColumn,
+		SenderRaceName: SenderRaceNameColumn,
 		SenderIP:       SenderIPColumn,
 		Subject:        SubjectColumn,
 		Body:           BodyColumn,