Adding get_balance() database function

[adamcharnock]

postgres.public> select get_balance(7)
[2024-06-28 20:25:18] 1 row retrieved starting from 1 in 430 ms (execution: 420 ms, fetching: 10 ms)

That 420ms for an account with 1 million legs.

This is on an M2 Macbook. Also, if I just calculate the balance for the one account (not including children) then it shaves about 30-40% off the execution time. This is a decent win, but I think we'll get bigger gains from adding running totals as per #76.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.18182% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.02%. Comparing base (81b90df) to head (d9f9071).
Report is 38 commits behind head on master.

Files	Patch %	Lines
hordak/utilities/db_functions.py	95.65%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage   93.01%   92.02%   -0.99%     
==========================================
  Files          59       66       +7     
  Lines        3836     4238     +402     
  Branches      249      274      +25     
==========================================
+ Hits         3568     3900     +332     
- Misses        224      284      +60     
- Partials       44       54      +10     

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