feat(docs): add OpenAPI 3.0 specification (#26)

[josealfredo79]

Closes #26

Summary

Added a formal OpenAPI 3.0.3 specification for the SmartDrop API, served via Swagger UI in development mode. The spec defines all current endpoints and includes planned future endpoints marked with x-draft: true.

Changes

• openapi.yaml — OpenAPI 3.0.3 spec at repo root with:
  • All endpoints from the issue (/health, /prices, /webhooks, /indexer/status, /ws)
  • JSON Schema definitions for all request/response bodies
  • Error schemas: 400, 401, 404, 422, 429, 500
  • Bearer token authentication in securitySchemes
  • x-rate-limit extension on rate-limited endpoints
  • Examples for every endpoint
  • Planned endpoints (/prices/batch, /indexer/status, /ws) marked with x-draft: true
• routes/apiDocs.js — Serves the raw YAML at /api-docs/openapi.yaml and Swagger UI at /api-docs (development only)
• index.js — Registers the /api-docs router
• api-docs.test.js — 7 unit tests: spec validation + Swagger UI reachability
• CI — Lint step with @redocly/cli lint openapi.yaml on every PR

Acceptance Criteria

• openapi.yaml at repo root with all endpoints
• Spec validates with @redocly/cli lint openapi.yaml (exit 0)
• Swagger UI reachable at /api-docs in development
• All error shapes documented
• Authentication scheme included
• CI step: lint the spec on every PR