chore: simplify backup repository creation

[nijel]

Proposed changes

• move it to the model
• simplify condition to check first for existing repository without hitting the database
• prepare for storing more info on backups

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 22 lines in your changes missing coverage. Please review.

Project coverage is 80.38%. Comparing base (cb78529) to head (7dc4ea2).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2362      +/-   ##
==========================================
+ Coverage   80.35%   80.38%   +0.03%     
==========================================
  Files          50       50              
  Lines        4423     4421       -2     
  Branches      425      424       -1     
==========================================
  Hits         3554     3554              
  Misses        719      719              
+ Partials      150      148       -2