fix(backend) api/iの未読のお知らせにcreatedAtを追加するように修正

[shiosyakeyakini-info]

What

api/iのunreadAnnouncementsにcreatedAtを再生成する処理を追加しました

Why

エンティティの定義と合わせるため　Related: #12091

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (42a3489) 79.38% compared to head (cc16e2c) 79.26%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #12092      +/-   ##
===========================================
- Coverage    79.38%   79.26%   -0.12%     
===========================================
  Files          932      932              
  Lines        99724    99728       +4     
  Branches      8135     8108      -27     
===========================================
- Hits         79161    79051     -110     
- Misses       20563    20677     +114     

Files	Coverage Δ
...ges/backend/src/core/entities/UserEntityService.ts	98.80% <60.00%> (-0.40%)	⬇️

... and 17 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[syuilo]

🙏🏻🙏🏻