test: add integration test for settlement endpoint

[probablyABug]

Description

Closes #183

Adds integration test coverage for the invoice settlement endpoint. The test verifies that settlement correctly distributes the invoice return pro-rata among all investors, transitions the invoice status to settled, prevents duplicate settlement attempts, and rejects settlement attempts from unauthorized sellers.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🎨 UI/UX improvement
• ♻️ Code refactoring
• ✅ Test addition or update
• 🔧 Configuration change

Checklist

• All GitHub Actions workflows are green on this PR (required for merge)
• Commit messages follow Conventional Commits (feat:, fix:, chore:, etc.) — enforced by CI
• No secrets, API keys, .env, or credentials committed (see CONTRIBUTING.md)
• My code follows the code style of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Testing

Added an integration test covering the complete settlement flow for a funded invoice with multiple investors.

How to Test

1. Seed a funded invoice with a face value of 9000 and three investors contributing 3000 each.
2. Authenticate as the invoice owner and call the settlement endpoint.
3. Verify each investor receives a return record for 3000.
4. Verify the invoice status changes to settled.
5. Attempt to settle the same invoice again and verify a 409 response with invoice_already_settled.
6. Authenticate as a different seller and attempt to settle the invoice, verifying that the request returns 403.

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• E2E tests added/updated (if applicable)
• Manual testing completed

Screenshots (if applicable)

Not applicable — this PR adds backend integration test coverage only.

Additional Notes

The test covers both the successful settlement path and important authorization/idempotency cases. It specifically verifies that multiple investors receive their correct pro-rata returns and that an already-settled invoice cannot be settled again.

For Reviewers

• Code quality and readability
• Test coverage
• Security implications
• Performance impact
• Breaking changes

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