feat(api): Moving contribution calculation out of models

[elliotcourant]

Right now monetr calculates contributions to expenses and goals using
some specific code co-located in the models package. This isn't great as
that package should not contain business logic as much as possible. This
is the groundwork of moving that logic out of that package and to make
use of the existing forecasting code to facilitate the same
calculations. This should also make things more consistent as both the
contribution and spending calculations will be the same code.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 27.72277% with 146 lines in your changes are missing coverage. Please review.

Project coverage is 52.29%. Comparing base (852db5f) to head (d6aa0fd).

Files	Patch %	Lines
server/forecast/contribution.go	0.00%	73 Missing ⚠️
server/forecast/spending.go	55.40%	31 Missing and 2 partials ⚠️
server/controller/forecast.go	0.00%	22 Missing ⚠️
server/forecast/forecast.go	45.45%	16 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1726      +/-   ##
==========================================
- Coverage   52.47%   52.29%   -0.19%     
==========================================
  Files         365      366       +1     
  Lines       20509    20630     +121     
  Branches      500      500              
==========================================
+ Hits        10763    10788      +25     
- Misses       9129     9222      +93     
- Partials      617      620       +3     

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