docs(indexer): public HTTP reference for the list API with a route-parity test

[jdjioe5-cpu]

What and why

Closes #35.
Provides a checked, readable public HTTP reference (\services/indexer/docs/API.md) for the indexer's list API so dashboard builders, payees, and pilot partners can integrate without inspecting TypeScript schemas. Accompanied by an automated route-parity vitest suite (\services/indexer/test/api-docs.test.ts) ensuring docs and Fastify routes stay permanently in lockstep.

Changes

• \services/indexer/docs/API.md\ (new): Covers all 5 routes (\GET /health, \GET /locks, \GET /locks/:id, \GET /payees, \GET /events/:txHash/:eventIndex). Documents request parameters (types, validation constraints, pagination), response shapes, nullability, HTTP error codes (400, 404, 503), decimal string amount scaling (explicitly noting minimal discrete unit \