fix: reject overflowing transfer amounts

[vycdev2]

Summary

• reject transfer amounts when adding the 5% fee would overflow decimal
• calculate transfer fees before opening a database transaction
• cover normal and overflowing transfer totals with regression tests

Verification

• dotnet test --no-restore --filter FullyQualifiedName~StocksServiceTests — passed: 7 tests
• dotnet build --no-restore — passed: 0 warnings, 0 errors
• dotnet test --no-build --no-restore — passed: 621 tests, 3 skipped
• git diff --check — passed

Risk

• Low. The change only rejects values that could not previously produce a representable transfer total.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.