feat(chain-watcher): implement Base chain watcher for USDC transfers

[coryli]

Summary

Add BaseChainWatcher that subscribes to ERC-20 Transfer events on the Base USDC contract via WebSocket, filtering for transfers to configured merchant deposit addresses.

Changes

• BaseChainWatcher class with WebSocket subscription to Transfer events via ConnectionManager
• TransactionStore interface with PostgreSQL and in-memory implementations
• Filter transfers by destination address (merchant deposit addresses from config)
• Parse transfer amount (6 decimals), sender, receiver, tx hash, block number
• Store discovered transactions in chain_transactions table
• Skip duplicate tx_hash insertions (idempotent)
• Updated index.ts to use BaseChainWatcher with environment configuration

How to Test

1. npm ci
2. npm run typecheck
3. npm test

For manual testing with real Base network:

export BASE_RPC_URL="https://mainnet.base.org"
export BASE_RPC_WS_URL="wss://mainnet.base.org"
export MERCHANT_ADDRESSES="0x..."
export DATABASE_URL="postgres://..."
npm run dev --workspace=@stripeonchain/chain-watcher

Tests

• Tests added/updated
• All tests pass (npm test)
• Lint passes (npm run lint)
• Type-check passes (npm run typecheck)
• Formatting passes (npm run format:check)

Checklist

• No secrets or credentials committed
• Migrations run cleanly up and down (npm run migrate:up / npm run migrate:down)
• Docker Compose starts without errors (docker compose up)

Related Issues

Closes #13

Made with Cursor