Add REST API wrapper for escrow creation

[crtahlin]

Description

Currently escrow creation requires full MCP SSE client implementation. Many integrators just want simple REST calls.

Current State

Must implement:

1. SSE connection to MCP server
2. Session management
3. Multiple round-trips for prepare/sign/submit

Proposed Solution

Add simple REST endpoints:

POST /api/v1/escrows/prepare
Body: { seller, contentHash, price, expiryDays, ... }
Returns: { unsignedTx, encryptionKey, estimatedGas }

POST /api/v1/escrows/submit  
Body: { signedTx }
Returns: { escrowId, txHash }

Benefits

• Dramatically simplifies integration
• Works with any HTTP client
• Still maintains security (signing happens client-side)
• Reduces barrier to entry for sellers

Alternative

At minimum, provide a Node.js SDK that wraps the MCP complexity.

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Found during seller flow UX testing

[crtahlin]

Partially Addressed by PR #38

PR #38 adds POST /api/v1/escrows endpoint that returns prepared transaction data:

POST /api/v1/escrows
Body: { action: "create", contentHash, keyCommitment, amount, ... }
Returns: { transaction: { to, data, value, chainId }, params, instructions }

This removes the MCP/SSE complexity - integrators can:

1. Call the REST endpoint to get encoded calldata
2. Submit the transaction via their wallet (MetaMask, ethers.js, etc.)

The /submit endpoint isn't needed since users submit transactions directly to the blockchain - the API just helps prepare the transaction data.

See: #38