feat(demo): add MVP RFQ backend using RFQ SDK

[0xMuang]

Build a small reference RFQ demo backend that uses the existing Corner Store RFQ SDK and connects it to the live Anvil/CLI demo flow.

The goal is not to provide a production RFQ server. The goal is to show that the Corner Store skeleton contracts, compliance engine, RFQ adapter, BUIDL-like ERC-3643 fixture, CLI, and RFQ SDK can be composed into a working end-to-end RFQ demo.

Scope

• Add a demo backend service that exposes a simple quote API.
  • GET /health
  • POST /rfq/quote
• Use services/rfq SDK internally to create RFQAdapter-compatible signed quotes.
• Use demo-only components:
  • fixed-rate pricing
  • in-memory nonce store
  • local demo maker signer
  • BUIDL-like asset / quote token / RFQAdapter addresses from config
• Connect the CLI to request a quote from the backend and fill it through the existing Router/RFQAdapter path.
• Update the demo runbook so the flow is easy to run locally.

Demo flow

1. Start local Anvil demo stack
2. Start RFQ demo backend
3. Setup investor / mock TA facts / KYC through CLI
4. CLI requests RFQ quote from backend
5. Backend signs quote using RFQ SDK
6. CLI submits quote through ExecutionRouter → RFQAdapter
7. Demo shows successful BUIDL-like RFQ trade
8. Demo also shows failure paths: unapproved maker, cancelled quote, non-compliant buyer

Acceptance criteria

• A developer can run the backend locally against the existing Anvil deployment artifact.
• The backend returns signed RFQ quotes compatible with RFQAdapter.
• CLI can request a backend quote and execute the RFQ fill path.
• BUIDL-like ERC-3643 demo asset works as the default regulated asset scenario.
• At least one happy-path RFQ trade and one compliance/maker failure path are documented and testable.
• Documentation clearly states that this is a reference demo backend, not a production RFQ server.

Out of scope

• Production RFQ server operation
• Real pricing engine
• Persistent nonce database
• KMS / production signer custody
• Real Securitize/TA integration
• Real BlackRock BUIDL integration
• NAV, redemption, or distribution rails
• Dashboard UI