[BE-30] Implement asset depreciation calculation endpoint

[yusuftomilola]

Overview

Implement an asset depreciation calculation endpoint that computes the current book value using straight-line depreciation inside backend/contrib/.

Location

All work must be inside backend/contrib/src/assets/.

Acceptance Criteria

• GET /assets/:id/depreciation endpoint implemented
• Calculates straight-line depreciation: (purchasePrice - salvageValue) / usefulLifeYears
• Default useful life: 5 years, default salvage value: 10% of purchase price
• Returns: { purchasePrice, currentBookValue, totalDepreciation, depreciationPerYear, ageInYears, percentDepreciated }
• Returns 400 if asset has no purchasePrice or purchaseDate
• Endpoint protected by JwtAuthGuard

[a-malik-gh]

@a-malik-gh has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would like to implement the asset depreciation calculation endpoint if assigned.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @a-malik-gh to this issue.

[drips-wave]

Congratulations, @a-malik-gh! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @a-malik-gh: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

This issue has been completed by @a-malik-gh as part of the Stellar Wave Program's 4th Wave 🥳

😎 @a-malik-gh: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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