feat: add public swap API server

[0xApotheosis]

Description

Adds a new @shapeshiftoss/public-api package that exposes swap functionality as a REST API for external partners.

Production URL: https://api.shapeshift.com
API Documentation: https://api.shapeshift.com/docs

Endpoints:

• GET /health - Health check
• GET /v1/assets - List all supported assets
• GET /v1/swap/rates - Fetch rates from all enabled swappers (0x, Relay, THORChain, etc.)
• GET /v1/swap/quote - Get executable quote with full transaction data
• GET /docs - Interactive Scalar API documentation

Architecture:

• Imports @shapeshiftoss/swapper directly - zero code duplication
• Server-side SwapperDeps with minimal EVM adapters fetching gas fees from Unchained
• esbuild bundling into single CJS file for deployment
• Multi-stage Docker build with BuildKit cache optimization
• Scalar-powered interactive API docs with zod-to-openapi

Note: Affiliate fee sharing is not yet implemented. Currently all swaps use the standard DAO affiliate fee (55 bps). Partner attribution, fee tracking, and revenue sharing will be added in Phase 2.

Issue (if applicable)

closes #

Risk

Low-Medium - This is a new standalone package/service that doesn't modify any existing swap flows in the web app.

What protocols, transaction types, wallets or contract interactions might be affected by this PR?

None directly. The public API reads from existing swapper infrastructure but doesn't introduce new on-chain transactions.

Testing

Engineering

# Health check
curl https://api.shapeshift.com/health

# Get rates (ETH -> USDC)
curl "https://api.shapeshift.com/v1/swap/rates?sellAssetId=eip155:1/slip44:60&buyAssetId=eip155:1/erc20:0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48&sellAmountCryptoBaseUnit=1000000000000000000"

# Interactive docs
open https://api.shapeshift.com/docs

Operations

• 🏁 My feature is behind a flag and doesn't require operations testing (yet)

This is a new standalone service deployed to Railway. It does not affect the main web application.

Screenshots (if applicable)

N/A - API service, no UI changes.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/public-swap-api

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