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/openapi.yaml

@@ -4400,41 +4400,6 @@ components:
         translator:
           type: string
           description: Identifier of the translation engine that produced the cached row.
-    UserMailSentSummary:
-      type: object
-      additionalProperties: false
-      required:
-        - message_id
-        - game_id
-        - kind
-        - body
-        - body_lang
-        - broadcast_scope
-        - created_at
-      properties:
-        message_id:
-          type: string
-          format: uuid
-        game_id:
-          type: string
-          format: uuid
-        game_name:
-          type: string
-        kind:
-          type: string
-          enum: [personal, admin]
-        subject:
-          type: string
-        body:
-          type: string
-        body_lang:
-          type: string
-        broadcast_scope:
-          type: string
-          enum: [single, game_broadcast, multi_game_broadcast]
-        created_at:
-          type: string
-          format: date-time
     UserMailInboxList:
       type: object
       additionalProperties: false
@@ -4445,6 +4410,13 @@ components:
           items:
             $ref: "#/components/schemas/UserMailMessageDetail"
     UserMailSentList:
+      description: |
+        Sender-side listing of personal messages authored by the
+        caller. Each item carries the same shape as inbox entries
+        (including the recipient snapshot); single sends contribute
+        one row per message, broadcasts contribute one row per
+        addressee so the in-game UI can collapse them by
+        `message_id` into a single stand-alone item.
       type: object
       additionalProperties: false
       required: [items]
@@ -4452,7 +4424,7 @@ components:
         items:
           type: array
           items:
-            $ref: "#/components/schemas/UserMailSentSummary"
+            $ref: "#/components/schemas/UserMailMessageDetail"
     UserMailUnreadCount:
       type: object
       additionalProperties: false