B-6: Multi-asset support (Stellar custom tokens / USDC)

[KuchiMercy]

Summary

The platform currently only supports XLM for escrow amounts. Stellar supports custom assets (including USDC via Stellar Asset Contract), and the architecture is ready for token abstraction. Adding multi-asset support significantly expands the platform's utility.

Requirements

• Extend the escrow creation DTO to accept an asset parameter: { code: string, issuer: string } (native XLM uses a sentinel value)
• Build an admin-managed token allowlist: AllowedAsset entity with fields for asset code, issuer, display name, icon URL, decimals, and active flag
• Add admin CRUD endpoints for the allowlist: POST/GET/PATCH/DELETE /admin/assets
• Integrate Stellar Horizon API to validate that an asset exists and fetch its metadata
• Update the Stellar integration service to handle SAC (Stellar Asset Contract) token transfers
• Update the smart contract interaction layer to pass asset info to the Soroban contract
• Add a GET /assets public endpoint returning the list of supported assets
• Update escrow amount display to respect asset decimals

Acceptance Criteria

• Escrows can be created with any asset on the allowlist
• XLM remains the default if no asset is specified (backward compatible)
• Admin can add/remove assets from the allowlist
• Asset validation checks the Stellar network before allowing creation
• USDC on Stellar works end-to-end (create, fund, release)
• Escrow amounts display with correct decimal precision per asset
• Invalid or deactivated assets are rejected at creation time

Context

• Escrow entity amount field: apps/backend/src/modules/escrow/entities/escrow.entity.ts
• Stellar integration: apps/backend/src/modules/escrow/services/escrow-stellar-integration.service.ts
• Stellar SDK: @stellar/stellar-sdk v13.3.0 (already in dependencies)
• Smart contract types: apps/onchain/src/types.rs

Rollout Phase

Phase 4 — User Features

Points: 200 (Difficult)