docs: add workflow scheduler section cleanly

[csmdg7]

Description

This PR introduces a comprehensive architectural overview of the asynchronous workflow automation engine inside docs/backend-architecture.md. It formally details how the backend manages task lifecycles through its WorkflowScheduler background loop. The documentation expands the baseline reference to break down:

1. Persistence models (SQLite state management updates via aiosqlite).
2. The continuous 5-second asynchronous evaluation tick sequence (_run_loop()).
3. Non-blocking task handoffs and execution boundaries using asyncio.create_task().

Related Issues

Closes #864

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Verified accuracy against implementation rules inside backend/secuscan/workflows.py.
• Conducted visual inspection using the Markdown Preview tool in VS Code to ensure clean rendering of headings, bullet blocks, and info/warning callouts.
• Cross-checked markdown linkage constraints to guarantee reference names precisely align with standard workspace structures.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Focused change with green checks; approving.