Extract WorkflowConfig embedded struct from module/api_handlers.go

[intel352]

Problem

Six related fields in the REST API handler struct (workflowType, workflowEngine, initialTransition, instanceIDPrefix, instanceIDField, seedFile) are always set together but live as flat top-level struct fields.

Fix Direction

Extract into an embedded WorkflowConfig struct:

type WorkflowConfig struct {
    Type             string
    Engine           string
    InitialTransition string
    InstanceIDPrefix string
    InstanceIDField  string
    SeedFile         string
}

Update all read/write sites to use the nested struct.

Context

See docs/DEFERRED_ISSUES.md — Code Smells section.