fix: consolidate credit tier resolution into CreditScoringService and…

[KingFRANKHOOD]

Summary

Removes the duplicate credit tier table in loans.service.ts and establishes CreditScoringService as the single source of truth for tier resolution, interest rates, and credit limits. All values now match the authoritative parameters-contract deployed on Stellar testnet.
Closes #100

Problem

There were two credit tier tables:

1. CreditScoringService (credit-scoring.service.ts) — proper NestJS service, injected but only used for loan assessment decisions
2. LoansService.mapScoreToCreditTier() (loans.service.ts:675) — private hardcoded table used by getAvailableCredit, contradicting both the service and the documented protocol parameters

Neither matched the contracts:

Tier	API (old)	Contracts
Gold	5,000	10,000 / 4%
Silver	3,000	5,000 / 6%
Bronze	1,500	2,500 / 8%
Starter	poor 500	1,000 / 10%

A learner could be shown a credit limit the chain would reject.

What Changed

Core fix

• Deleted mapScoreToCreditTier() from loans.service.ts — no longer exists
• Wired getAvailableCredit() through CreditScoringService.resolveTier() — both paths now use the same service
• Added CREDIT_TIERS constant array in credit-scoring.service.ts — one table, one place, named constants matching the parameters-contract
• Added CreditScoringService.resolveTier(score) method — public, canonical tier resolution

Tier naming reconciliation

• Changed tier name from poor to starter across all DTOs, services, and consumers to match the contracts
• BREAKING: API responses now return starter instead of poor for the lowest tier

Contract-aligned values

• Gold: maxCredit 10,000, interestRate 4% (was 5,000)
• Silver: maxCredit 5,000, interestRate 6% (was 3,000)
• Bronze: maxCredit 2,500, interestRate 8% (was 1,500)
• Starter: maxCredit 1,000, interestRate 10% (was 500)

Consumers updated

• ReputationService.mapToReputation() now uses CREDIT_TIERS instead of inline if/else
• ReputationTier type: poor -> starter

Tests

• 36 credit-scoring tests including contract alignment assertion and boundary scores 90, 89, 75, 74, 60, 59
• 47 loans tests pass with updated credit limits
• 11 reputation tests pass with updated tier names and rates

Breaking Changes

StepFi-Web and StepFi-App must update:

1. Replace poor tier name references with starter in tier-based rendering logic
2. Update credit limit display: gold 10,000, silver 5,000, bronze 2,500, starter 1,000
3. Update interest rate display: gold 4%, silver 6%, bronze 8%, starter 10%

Files Changed

File	Change
src/modules/credit-scoring/credit-scoring.service.ts	Added CREDIT_TIERS, CreditTier, resolveTier()
src/modules/loans/loans.service.ts	Deleted mapScoreToCreditTier(), wired to CreditScoringService
src/modules/reputation/reputation.service.ts	Uses CREDIT_TIERS instead of inline tier mapping
src/modules/reputation/dto/reputation-response.dto.ts	poor -> starter
src/modules/loans/dto/available-credit-response.dto.ts	poor -> starter
test/unit/modules/credit-scoring/credit-scoring.service.spec.ts	Comprehensive tier + contract alignment tests
test/unit/modules/loans/loans.service.spec.ts	Updated mock values and expectations
test/unit/modules/reputation/reputation.service.spec.ts	Updated tier names and values
test/e2e/modules/loans/loans.e2e-spec.ts	Updated tier name
test/e2e/modules/reputation/reputation.e2e-spec.ts	Updated tier name

CI

• npm run build — 0 errors
• npm run lint — 0 errors
• npm test — all 94 unit tests pass