docs: strategy for scaling HUF execution backends

[esafwan]

Summary

Adds a detailed scaling strategy document for HUF execution modes and deployment patterns.

What changed

• Added docs/SCALING_HUF_STRATEGY.md
• Linked the new scaling strategy from README.md

Covered in the document

• Problem definition for scaling HUF beyond direct Frappe/MariaDB execution
• Three execution backends:
  • Frappe Direct API
  • Frappe Redis Queue / Background Job
  • Temporal Executor
• When each mode should be used
• Example API behavior for each mode
• Mermaid deployment diagrams for each mode
• White Umbrella / Umbrella usage logic and recommended execution mode mapping
• Temporal huf-executor concept, responsibilities, non-responsibilities, process model, and build plan
• Frappe Cloud + external Temporal deployment model

Notes

This is documentation only. No runtime code or migration changes are included.