[SC] Score Gatekeeper PoC: enforce min tier in lending txs

[Cbiux]

Summary

Score Gatekeeper PoC: Soroban contract or middleware that reads ZCore on-chain tier via get_score and rejects transactions below a minimum tier (e.g. Blend borrow requires Tier B+).

Proves portable credit gating for #18 Blend integration.

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Background

• Score registry: Contracts/score-registry/ (Deploy score-registry Soroban contract to Stellar testnet #16 deploy required)
• Interface: Contracts/interfaces/README.md, Server/src/types/soroban/score-types.ts
• Mock for tests: Contracts/mock-score-registry/

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Proposed PoC

Option A — Soroban gatekeeper contract:

• require_min_tier(wallet, min_tier_code) → panic if get_score(wallet).tier < min
• Invoked as sub-call from lending pool adapter

Option B — Server-side pre-check (faster PoC):

• POST /api/user/request already evaluates lender eligibility
• Extend with public GET /api/user/{wallet}/tier for protocols

Start with Option B if #16 not deployed; add Option A when contract ID is live.

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Deliverables

Path	Action
Docs/integrations/score-gatekeeper.md	Tier gating flow, tier codes 0–3
Server/src/controllers/user.controller.ts	Optional GET .../tier if needed
Contracts/gatekeeper/	Optional Soroban PoC

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Acceptance criteria

• Documented flow: wallet below min tier → rejected with clear error
• Test wallet above tier → allowed
• Uses tierCodeToLabel / on-chain read — no duplicate scoring logic
• Works with mock contract in CI (withMockContractId)

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Related

• Deploy score-registry Soroban contract to Stellar testnet #16 testnet deploy
• Blend Protocol integration: loan_repaid events on testnet #18 Blend integration