fix(backend): deleted remote users could be shown in users/show endpoint

[zyoshoka]

What

users/show エンドポイントに削除したリモートユーザーを指定しても取得できてしまう問題を修正

Why

Closes #14728

Additional info (optional)

他のエンドポイントでも塞ぐべきでしょうがそもそも isSuspended についてもそうなっていない、というケースが多いようなのでひとまず users/show エンドポイントのみの対応としています

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

[github-actions]

このPRによるapi.jsonの差分
差分はありません。
Get diff files from Workflow Page

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 38.54%. Comparing base (12bc671) to head (08a6260).

Files with missing lines	Patch %	Lines
...ges/backend/src/server/api/endpoints/users/show.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #14752      +/-   ##
===========================================
- Coverage    39.66%   38.54%   -1.13%     
===========================================
  Files         1553     1553              
  Lines       195050   195051       +1     
  Branches      2574     3320     +746     
===========================================
- Hits         77363    75175    -2188     
- Misses      117119   119269    +2150     
- Partials       568      607      +39     

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