Implement Money-Rails

[D0gmaDev]

Closes #478

Implement proper money handling with money-rails

This PR introduces the money-rails gem to handle monetary values properly throughout the application, replacing manual cents manipulation with Money objects.

Core Implementation:

• Added money-rails gem with EUR as default currency
• Migrated price columns to price_cents (integer storage for cents)
• Added monetize macro to Article and SubscriptionOffer models
• Implemented total_price method for both Sale and Refund models

Improvements:

• All monetary arithmetic now uses Money objects (no more manual float operations)
• Currency consistency validation on all price fields
• Consistent price display using humanized_money_with_symbol
• JavaScript layer properly documented to use cents

Invoice System:

• Enhanced InvoicePdfGenerator to handle both Money objects and serialized Hash formats
• Added to_money and format_money helper methods for PDF generation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3d58786) to head (098207d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #534   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          109       109           
  Lines         2359      2388   +29     
  Branches        72        78    +6     
=========================================
+ Hits          2359      2388   +29     

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