Promotions: Specify inverse_of/dependent options

[mamhoff]

Summary

This adds inverse_of and dependent options to has_many relation declarations in the promotions gem.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.95%. Comparing base (3333477) to head (a017a24).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6274      +/-   ##
==========================================
+ Coverage   86.63%   88.95%   +2.32%     
==========================================
  Files         520      860     +340     
  Lines       11946    18423    +6477     
==========================================
+ Hits        10349    16388    +6039     
- Misses       1597     2035     +438     

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