Add OpenAPI spec for toll-booth payment endpoints

[TheCryptoDonkey]

toll-booth exposes several HTTP endpoints (/create-invoice, /invoice-status/:paymentHash, /nwc-pay, /cashu-redeem) but there is no machine-readable API specification. An OpenAPI document would help integrators understand the request/response shapes without reading source code, and could eventually power client SDK generation.

Scope

• Create docs/openapi.yaml describing the four payment endpoints
• Document request bodies, response shapes, and error codes (402, 400, 404, 200)
• Include the Authorization: L402 <macaroon>:<preimage> header schema
• Reference the existing types in src/core/types.ts for request/response shapes

Helpful context

• src/core/create-invoice.ts - invoice creation handler (request body: { tier?, route? }, response: { paymentHash, bolt11, macaroon })
• src/core/invoice-status.ts - status polling handler (response: { paid, preimage?, creditBalance? })
• src/core/nwc-pay.ts - NWC payment handler
• src/core/cashu-redeem.ts - Cashu redemption handler
• src/core/types.ts - all request/result type definitions
• Estimated effort: 2-3 hours