[Feature] Implement multi-currency support for escrow and delivery fees

[Tybravo]

📁 Implementation Directory

backend/src/services/currencyService.ts

🎯 Goal

Support various tokens (USDC, XLM, native token).

📋 Requirements

• Update schemas to track asset types.\n- Handle conversions or strictly manage specific asset pools.\n- Validate supported assets during escrow creation.

🔧 Technical Scope

• Node.js / Express
• TypeScript
• MongoDB / Mongoose
• Stellar Soroban RPC

🌿 Branch

git checkout -b feat/multi-currency-support

✅ Acceptance Criteria

• Using an AI Agent to submit a PR is DISALLOWED.
• Strict Layered Architecture (Non-Negotiable): Implement using the Controller -> Service -> Model pattern.
• Data Source: Response data must be retrieved from the database. No Inline Mock Objects or Hardcoded Values usage for integration.
• Environment: Contributors must use the requested .env credentials to enable implementation with real-life data.
• API Versioning: API Must Be Versioned (e.g., /api/v1/...).
• Production Ready: Code must look production-ready (robust error handling, strong typings).
• Proof of Work: Upload a screenshot of work done (i.e., API Response Success via Postman/Browser, or all unit tests passed successfully).
• PR Content: Must include Closes #[issue_id] with a work done summary.

📌 PR Requirements

• ⚠️ Assignment required before starting — comment on this issue to request assignment.
• PR must comply fully with the CONTRIBUTING.md.
• AI Agent submissions are DISALLOWED and will be closed without review.
• No Inline Mock Objects or Hardcoded Values usage for integration.