Clear search cache

[CrazyTim71]

This PR introduces an “Advanced” section in the settings page, where users can clear all cached search entries with a single click.
While cached searches normally expire automatically after 24 hours, it can be useful to refresh search results immediately.
For example, if a book isn’t found, I'll add it to Hardcover within minutes and then want updated search results without waiting the full expiration period.

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.22%. Comparing base (266c818) to head (234b1bd).
⚠️ Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/users/views.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #811      +/-   ##
==========================================
- Coverage   83.26%   83.22%   -0.05%     
==========================================
  Files          69       69              
  Lines        6820     6831      +11     
==========================================
+ Hits         5679     5685       +6     
- Misses       1141     1146       +5     

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