Add refund received timestamp column, record time when marking refund

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mail_api.py

@@ -1278,10 +1278,11 @@ async def toggle_refund_received(email: str) -> ApiResponse:
         status = await db_manager.get_claude_payment_status(email)
         current = status.get('refund_received', '0') if status else '0'
         new_val = '0' if current == '1' else '1'
-        ok = await db_manager.update_claude_payment_note(email, refund_received=new_val)
+        now = datetime.now().strftime('%Y-%m-%d %H:%M:%S') if new_val == '1' else ''
+        ok = await db_manager.update_claude_payment_note(email, refund_received=new_val, refund_received_at=now)
         if ok:
             await cache.invalidate_payment()
-            return ApiResponse(success=True, data={"refund_received": new_val})
+            return ApiResponse(success=True, data={"refund_received": new_val, "refund_received_at": now})
         return ApiResponse(success=False, message="更新失败")
     except Exception as e:
         logger.error(f"切换退款状态失败: {e}")