fix: enable sqlite wal mode

[a1denvalu3]

Summary

• Enables SQLite journal_mode=WAL and busy_timeout=5000 to improve database concurrency.

Resolves #907

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 62.96%. Comparing base (89564fe) to head (4814bbb).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
cashu/core/db.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #908      +/-   ##
==========================================
+ Coverage   61.51%   62.96%   +1.45%     
==========================================
  Files          93       93              
  Lines       11051    11052       +1     
==========================================
+ Hits         6798     6959     +161     
+ Misses       4253     4093     -160     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[prusnak]

Do we know why the tests fail?

[a1denvalu3]

Do we know why the tests fail?

A read of a quote in one test was racing with a write. Added a delay and now it seems to work.