[DOCS] Add workflow scheduler section to backend architecture guide

[utksh1]

Summary

The backend architecture guide should describe workflow persistence, scheduling, and queue interaction at a high level.

Problem

The current documentation is either missing a critical explanation, outdated, or fragmented across too many places.

Where to update

• Suggested files: docs/backend-architecture.md, backend/secuscan/workflows.py
• Related sections or headings: the sections implied by the title and summary

Expected outcome

A contributor should be able to complete this update with a focused documentation PR and leave the surrounding docs easier to follow.

Acceptance criteria

• The affected docs are updated
• Steps, commands, and references are accurate
• Cross-links are added where they reduce ambiguity
• A maintainer can follow the updated guidance without extra clarification

Verification

Review the changed docs against the current code paths or scripts they describe and spot-check any commands or file references.

Additional context

Prefer precise edits over broad rewrites unless the referenced section is already structurally broken.