fix(chat): update unlock response contract

docs/backend/unlock.md

@@ -0,0 +1,32 @@
+用途：解锁某一条历史里的付费/私密消息。
+请求体：
+{
+  "messageId": "消息ID"
+}
+需要带登录 token：
+Authorization: Bearer <token>
+Content-Type: application/json
+返回示例：
+{
+  "code": 200,
+  "message": "success",
+  "success": true,
+  "data": {
+    "unlocked": true,
+    "content": "完整消息内容",
+    "reason": "ok",
+    "creditBalance": 9180,
+    "creditsCharged": 10,
+    "requiredCredits": 10,
+    "shortfallCredits": 0,
+    "lockDetail": {
+      "locked": false,
+      "showContent": true,
+      "showUpgrade": false,
+      "reason": null,
+      "hint": null,
+      "detail": null
+    }
+  }
+}
+如果积分不够，会返回 unlocked:false，并带 requiredCredits / shortfallCredits / creditBalance。