Reorder pricing plans

🔍 Problem Statement

As a Pelcro admin, plans display in a fixed creation-order on both the Platform UI and the Plan Selection Modal (Elements UI), which gives customers an unintuitive sequence — especially when multiple tiers or region-specific plans exist.

💡 User Story

As an admin, I want to drag-and-drop plans into a custom order from Products → Plans, so the order I set is exactly what customers see in every plan-selection surface.

🎯 Definition of Done (DoD)

• ✔ Given the Platform → Products → Plans page, when an admin drags a plan into a new position, then the new order is auto-saved (no Save button) and persisted at the plan level.
• ✔ Plan Modal, Change-Plan flow, and Checkout plan selector all respect sort_order from the API; no caching conflicts.
• ✔ This change will impact UI / API / SDK — specifically the Plans page (Platform UI), the plans Core API (new sort_order field), and the ReactJS Elements Plan Modal component.
• ✔ Limitations: plans without sort_order default to creation order (backward-compatible).