Implement Stellar CCTP service layer: burn, attest, and mint flow

[grantfox-development]

Overview

Implement the core service layer that orchestrates the CCTP burn → attest → mint cycle between an external chain (e.g., Ethereum) and Stellar using Soroban smart contracts.

Background

CCTP works in three phases:

1. Burn – USDC is burned on the source chain and a message is emitted.
2. Attest – Circle's Attestation API signs the message.
3. Mint – The signed message is submitted to the destination chain (Stellar) to mint USDC.

Tasks

• Create CCTPService class under src/modules/cctp/
• Implement initiateBurn(amount, sourceChain, destinationAddress) — submits burn tx and stores the resulting message hash
• Implement pollAttestation(messageHash) — polls Circle Attestation API until status: complete
• Implement mintOnStellar(attestation, messageBytes) — calls the Soroban receiveMessage entry point
• Persist transfer state transitions to the database (pending → attesting → minting → complete → failed)
• Add exponential backoff retry logic for attestation polling
• Integrate with existing WalletService / Stellar RPC client

Acceptance Criteria

• CCTPService unit-tested with mocked Circle API and Stellar RPC
• Transfer state correctly persisted and queryable
• All three phases covered by integration tests against Stellar testnet
• No secrets or private keys hardcoded — all via env vars