Adopt axum + OpenAPI/Swagger for API documentation

[jdhoffa]

Summary

Migrate the current HTTP API implementation to an ecosystem-idiomatic Rust stack using axum for routing and utoipa + utoipa-swagger-ui for OpenAPI and Swagger docs.

Motivation

• Keep API implementation aligned with common Rust ecosystem patterns.
• Provide self-documenting API contracts for frontend and external consumers.
• Make endpoint behavior and schemas easier to validate and evolve.

Proposed scope

• Replace custom HTTP server with axum.
• Generate OpenAPI spec from Rust types/handlers using utoipa.
• Serve docs endpoints:
  • GET /openapi.json
  • GET /docs (Swagger UI)
• Preserve existing API behavior:
  • GET /state
  • GET /telemetry?from=&to=
• Keep telemetry schema v1 field names unchanged.

Acceptance criteria

• API server runs with --api-bind as before.
• /openapi.json returns valid OpenAPI JSON.
• /docs serves Swagger UI successfully.
• Existing integration coverage continues to pass for /state and /telemetry.
• Add/adjust integration tests to validate docs endpoint availability.
• README documents docs endpoints and usage.

Notes

This is intentionally deferred work; issue is for backlog prioritization.

[jdhoffa]

relates to #22 and #72