[Testing] Add unit tests for scoreDecayService inactivity decay logic

[Akpolo]

https://t.me/+DOylgFv1jyJlNzM0

Why this matters

scoreDecayService.ts (with cron/scoreDecayJob.ts) decays credit scores for inactive borrowers (#535). The job wrapper has a test (cron/__tests__/scoreDecayJob.test.ts) but the service that computes WHICH users decay and by HOW MUCH is untested. Credit-score mutation is financially sensitive — an off-by-one in the inactivity window or decay amount silently penalizes active users or never decays inactive ones.

Acceptance criteria

• Add scoreDecayService tests covering: user inactive past the threshold decays by the configured amount, active user does not decay, decay floored at the minimum score, and batch processing across multiple users
• Assert score-floor and idempotency (running twice in the same window does not double-decay)
• Mock the DB/contract layer consistently with existing service tests

Files to touch

• backend/src/services/__tests__/scoreDecayService.test.ts (new)

Out of scope

• Changing decay parameters or schedule
• Contract-side score logic

[ExcelDsigN-tech]

@ExcelDsigN-tech has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I'm a full-stack developer with hands-on experience across Next.js, Node.js, NestJS, React, and TypeScript, and I also specialize in smart contract development using Rust and Soroban. I build end-to-end, from polished frontends to robust backends to on-chain logic, with a strong focus on clean, maintainable code that works in production, not just in isolation. I communicate progress consistently and keep things fully transparent throughout. I'm confident I can deliver within 12 hours.

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