Fix locked balance allocation

[koutst]

This PR fixes how locked vs. unlocked RSC balance is tracked during fundraise contributions and refunds

• Locked balance contributions were being refunded as unlocked.
• Add purchase to balances to reference on refund.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.63%. Comparing base (1191eb3) to head (85f289e).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/purchase/services/fundraise_service.py	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3211      +/-   ##
==========================================
+ Coverage   78.62%   78.63%   +0.01%     
==========================================
  Files         614      614              
  Lines       34415    34437      +22     
==========================================
+ Hits        27059    27081      +22     
  Misses       7356     7356              

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