[Documentation] API documentation with OpenAPI/Swagger

## API Documentation Checklist

- [ ] Add FastAPI/OpenAPI schema generation for agent server
- [ ] Add Swagger UI / ReDoc endpoints
- [ ] Document all bot commands
- [ ] Document agent registration API
- [ ] Document agent command API
- [ ] Document SNMP device API
- [ ] Document dashboard WebSocket API
- [ ] Add API versioning strategy
- [ ] Generate markdown docs from OpenAPI spec