Refactor the reference Order Book to consume Signed Orders

[gustavobftorres]

Parent

LIME-Protocol/lime-onchain#3

What to build

Update the backend Matching Engine so it receives Signed Orders, matches compatible Buy and Sell Orders, and submits Trade Execution through the official SDK rather than trusting backend-created Orders.

Acceptance criteria

• API accepts Signed Orders from clients and stores them as Order Book entries
• Matching only pairs Orders that satisfy side, Market, Price, Quantity, and expiration rules
• Backend calls settleSignedOrders from @lime/solana for Trade Execution
• Backend treats on-chain validation as the source of truth for successful Trade Execution
• Tests cover happy-path match and rejected invalid, expired, replayed, or out-of-range Orders

Blocked by

• https://github.com/LIME-Protocol/lime-sdk/issues/4