FTTを無効化する際にキャッシュを削除するように

[penginn-net]

What

FTT有効→無効に切り替えた際にFTTキャッシュを削除するようにしました

Why

Fix #12126
#12126 が発生するため流速の遅いサーバーでは有効→無効→有効とするとタイムスリップしてしまうため
(有効→無効したときにRedisのメモリ占有を減らせるはず？)

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

[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 22.85714% with 27 lines in your changes missing coverage. Please review.

Project coverage is 40.00%. Comparing base (3c81926) to head (bb1a802).

Files with missing lines	Patch %	Lines
packages/backend/src/core/FanoutTimelineService.ts	15.62%	27 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #15078      +/-   ##
===========================================
+ Coverage    39.96%   40.00%   +0.04%     
===========================================
  Files         1563     1563              
  Lines       197878   197910      +32     
  Branches      3646     3665      +19     
===========================================
+ Hits         79080    79178      +98     
+ Misses      118193   118159      -34     
+ Partials       605      573      -32     

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