Feat/workflow state machine(#280)

[JohnOluB]

PR description

Summary

closes #280

Implements server-side state machine logic for the EventHorizon
visual workflow editor using XState v5.

What's changed

File	Purpose
src/services/workflowMachine.js	XState v5 machine — 7 states, retry back-off, validation
src/services/workflow.service.js	Actor lifecycle manager; persists every snapshot to MongoDB
src/models/workflowExecution.model.js	Mongoose schema for execution history
src/controllers/workflow.controller.js	Express handlers (start, get, history, send event, cancel)
src/routes/workflow.routes.js	REST routes with OpenAPI JSDoc, using existing auth/permission middleware
tests/unit/workflowMachine.test.js	19 unit tests (Node built-in runner, no extra deps)
tests/integration/workflow.service.test.js	8 integration tests with stubbed MongoDB
docs/workflow-state-machine.md	Full documentation

State flow

idle → validating → pending → executing → completed
↘ failed ↘ retrying → executing
↘ failed (retries exhausted)

[drips-wave]

@JohnOluB Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[EDOHWARES]

Nice implementation, lgtm!