[BE-031] Implement off-chain real-time yield calculator engine

[ONEONUORA]

Description

Build utility to calculate estimated yield earnings up-to-the-second for UI display.

Files to Edit/Create

• backend/src/services/yield_calc.rs (new file)

Acceptance Criteria

• Estimate interest accrued since last blockchain event.
• Expose helper function for feed query responses.

Guidance / Hints

Compute linear interest using user's current balance and elapsed time since last sync.

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[aliyuHabibu]

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[josephchimebuka]

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[josephchimebuka]

Implemented in #426 — will close this issue when merged.

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