fix: use row lock select statements

[a1denvalu3]

Summary

• Improved database concurrency by using row locks (select statements locks) more heavily for PostgreSQL.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.06%. Comparing base (89564fe) to head (989d0bc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #906      +/-   ##
==========================================
+ Coverage   61.51%   63.06%   +1.55%     
==========================================
  Files          93       93              
  Lines       11051    11053       +2     
==========================================
+ Hits         6798     6971     +173     
+ Misses       4253     4082     -171     

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[a1denvalu3]

We should also consider removing the lock on proofs_pending when we insert new entries and let the uniqueness constraint do its job.