add child questions backend code

[MaHaWo]

• add ChildQuestion and ChildAnswer database models
• add default child questions which are added when the database is initialized
  • name
  • birth date
  • remarks
  • optional file upload for image
    These are intended to fill the Child model upon registration of a new child. They have fixed question-ids of 1 to 4 in the ChildQuestion table in the above order.
• add tests for new code
• unify model classes via inheritance for questions, answers where easily possible, i.e., where no sql tables are involved.
• sonarcloud complaint is about about duplication in autogenerated code. Not sure if this should be adressed.

a substantial chunk of the changed LoC is autogenerated openapi code, so this PR isn´t quite as big as it seems.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.53125% with 7 lines in your changes missing coverage. Please review.

Project coverage is 29.21%. Comparing base (06f1284) to head (e0a16f4).

Files with missing lines	Patch %	Lines
...backend/src/mondey_backend/databases/milestones.py	25.00%	6 Missing ⚠️
mondey_backend/src/mondey_backend/routers/users.py	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   26.83%   29.21%   +2.38%     
==========================================
  Files          93       93              
  Lines        3455     3550      +95     
  Branches      122      122              
==========================================
+ Hits          927     1037     +110     
+ Misses       2461     2446      -15     
  Partials       67       67              

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[sonarcloud]

Quality Gate failed

Failed conditions
6.6% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud