Phase 28 (Step 3a): /sent returns full message detail per recipient

Phase 28's in-game mail UI threads sent messages by the recipient
race name, so the bulk `/sent` endpoint now returns the same
`UserMailMessageDetail` shape as `/inbox` — single sends contribute
one row per message, broadcasts contribute one row per addressee
and the UI collapses them by `message_id` into a stand-alone item.

- `Store.ListSent` / `Service.ListSent` switched from `[]Message`
  to `[]InboxEntry`. SQL grows an INNER JOIN with
  `diplomail_recipients`.
- Handler emits `userMailMessageDetailWire` items; the deprecated
  `userMailSentSummaryWire` is removed.
- `openapi.yaml`: `UserMailSentList.items` now reference
  `UserMailMessageDetail`; the standalone `UserMailSentSummary`
  schema is dropped.

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

backend/internal/diplomail/service.go

@@ -316,10 +316,13 @@ func (s *Service) allowedKinds(ctx context.Context, gameID, userID uuid.UUID) (m
 	return map[string]bool{KindAdmin: true}, nil
 }
 
-// ListSent returns personal messages authored by senderUserID in
-// gameID, newest first. Admin/system rows have no `sender_user_id`
-// and are therefore excluded; the user surface does not need them.
-func (s *Service) ListSent(ctx context.Context, gameID, senderUserID uuid.UUID) ([]Message, error) {
+// ListSent returns the sender-side view of personal messages
+// authored by senderUserID in gameID, newest first. Each entry pairs
+// the message with one of its recipient rows; single sends contribute
+// one entry per message, broadcasts contribute one entry per
+// addressee. Admin and system rows have no `sender_user_id` and are
+// therefore excluded; the user surface does not need them.
+func (s *Service) ListSent(ctx context.Context, gameID, senderUserID uuid.UUID) ([]InboxEntry, error) {
 	return s.deps.Store.ListSent(ctx, gameID, senderUserID)
 }