Implement workflow system for semaphore ui

[fiftin]

SONNET 4.5

Add a Workflow feature with a visual editor and execution engine to enable multi-step automation pipelines.

This PR introduces a complete workflow orchestration system, including new database models, a REST API, a robust execution engine, and a Vue.js frontend for visual editing and real-time monitoring. It allows users to define and run complex, multi-step automation processes with sequential, parallel, and conditional logic, addressing a critical missing feature in Semaphore UI.

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[cursor[bot]]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch. _{Learn more about Cursor Agents}