Implement oracle-backed collateral valuation

[Cedarich]

Summary

Calculate borrowing power using asset prices rather than raw deposited amounts.

Acceptance Criteria

• Price inputs are available to the contract or read model strategy.
• Collateral checks use configurable oracle-backed values.
• Borrow constraints are validated in tests.

Campaign Context

GrantFox OSS campaign issue for building VeilLend on Stellar.

[MaryammAli]

hi, maintainer i would like to tackle this issue
kindly assign this to me

[grantfox-oss]

🦊 GrantFox — @MaryammAli has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #31)
2. Your PR will be reviewed by the Zyntarivoid maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

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

@MaryammAli's PR #103 was approved and merged by @Cedarich.

🏆 @MaryammAli: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

👏 Great work, @MaryammAli! Keep contributing to Zyntarivoid.