feat(settlement): add debt simplification preview

[thelight08]

Closes #271

Exposes the existing greedy debt simplification logic through a group settlement preview endpoint, preserving the selected asset and issuer on every operation.

[mergekeeper]

Needs review

Linked to #271, but the diff does not match the issue scope.

The pull request introduces unrelated changes to audit-log routes which are outside the scope of issue #271.

Reviewed commit: 7bafc8873adfea1f1be3ef466c056d3f73e4dd54.

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[thelight08]

@Cjay-Cyber-2 The debt simplification preview endpoint is pushed and ready for review. It preserves asset code and issuer on every suggested operation. Closes #271.

[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #271.
Verdict: clean

The debt simplification algorithm wrapper and preview endpoint were correctly added in accordance with the issue requirements.

Reviewed commit: 2521565c02a97a6e9a1cd19d03f89ff4ca182b71.
CI and merge eligibility are checked separately.

[thelight08]

✅ Merge conflicts with main have been resolved and the branch is up to date with the latest base. @mergepay this PR is ready for review. It closes issue #271.