Add index to created_at on attachment and pictures tables

[tvdeyen]

What is this pull request for?

We have the recent scope on the attachment and pictures models. Also the picture archive now defaults to sorting by recent images.

On large projects it should be a good idea to have an index on these columns.

It will add the index concurrently on Postgres.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.14%. Comparing base (7e196d6) to head (a94c7dc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3370   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files         277      277           
  Lines        7248     7248           
=======================================
  Hits         7041     7041           
  Misses        207      207           

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