refactor(module): extract WorkflowConfig embedded struct from RESTAPIHandler (closes #69)

Six workflow-related fields (Type, Engine, InitialTransition, InstanceIDPrefix,
InstanceIDField, SeedFile) that are always set together are extracted into a
dedicated WorkflowConfig struct, which is then embedded in RESTAPIHandler.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: intel352