backend: Implement Loan Lifecycle & Collateral Management Endpoints

[ONEONUORA]

Problem Description

To support yield-bearing inheritance vaults integrated with Soroban lending pools, the frontend API service defines functions (freezeLoans, recallLoans, liquidateAndSettle) that call:

• POST /api/plans/{id}/freeze-loans
• POST /api/plans/{id}/recall-loans
• POST /api/plans/{id}/liquidate-settle

None of these routes are registered in backend/src/api.rs or connected to the Soroban lending contract.

Missing Endpoints & Specs

• Implement POST /api/plans/{id}/freeze-loans to halt new borrowing against vault collateral when a plan enters the triggered state.
• Implement POST /api/plans/{id}/recall-loans to invoke Soroban cross-contract calls recalling loaned capital and accrued yield back into the inheritance contract vault.
• Implement POST /api/plans/{id}/liquidate-settle to trigger collateral liquidation if loan recall fails or defaults during settlement.
• Include proper authorization checking (Signature or JWT Auth) and emit status update events over WebSocket.

Affected Files

• backend/src/api.rs
• frontend/app/lib/api/plans.ts
• backend/src/stellar_submit.rs

[GideonBature]

Greetings maintainer, please I will love to work on this issue.

Thank you for your consideration.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1035)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@GideonBature's PR #1054 was approved and merged by @ONEONUORA.

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👏 Great work, @GideonBature! Keep contributing to Fracverse.