feat(rfq): add local RFQ backend and headless demo flow

[0xMuang]

Summary

Build the local RFQ backend path needed to exercise RFQ end-to-end without a dashboard: request a quote through a reference service, receive a signed EIP-712 RFQ quote, and use it in a local/headless fill flow against the existing RFQAdapter.

This follows PR #21, which implements the onchain RFQ settlement adapter and the minimal quote signer library. This issue should make that path easy to run locally, but it is still reference/demo infrastructure, not a production RFQ dealer.

Scope

• Add a small HTTP API under services/rfq
• Provide a POST /quotes endpoint that validates request input and returns a signed RFQ quote
• Reuse the existing RFQQuoteService, EIP-712 typed-data builder, expiry handling, nonce handling, and unsafe-number guards
• Add deterministic local config for chainId, verifyingContract, default TTL, maker/dealer address, venue address, and signer mode
• Add a minimal signer abstraction suitable for local/reference use
• Add a headless local demo flow or script that can:
  • start from known local deployment/config values
  • request a quote from the RFQ backend
  • submit the returned quote/signature to the Router/RFQAdapter path
  • verify success/failure and balances in tests or scripted output
• Add API and headless flow smoke/integration tests
• Document local usage in README or services/rfq docs
• Keep scripts/check.sh and CI aligned if service commands change

Explicit follow-ups / not in this issue

• Dashboard or browser UI for quote/fill visualization
• BUIDL-like ERC-3643 demo asset fixture
• Real BUIDL mainnet integration
• Production pricing engine
• Dealer inventory management
• Custody or escrow
• Partial fill
• Quote cancellation beyond a documented follow-up design seam
• Websocket / quote discovery
• Production key custody or HSM integration
• Real KYC / ACE / ONCHAINID / issuer claim integration
• Compliance decision-making in the backend

Acceptance Criteria

• A local caller can request a quote through the API and receive the same quote/signature shape that RFQAdapter verifies
• Invalid addresses, unsafe numeric inputs, invalid TTL, and malformed quote requests are rejected before signing
• A headless local test/script can exercise quote request → signed quote → Router/RFQAdapter fill using local config
• The backend remains reference-only and does not claim production dealer, custody, pricing, or compliance authority
• Existing RFQ service tests still pass, with added API/headless flow coverage if introduced
• Repository docs clearly distinguish RFQ settlement adapter, reference quote signer, local RFQ backend, and future dashboard/demo asset work

Verification

• cd services/rfq && npm test
• scripts/check.sh
• git diff --check