REST API for All PRO Modules

Comprehensive REST API coverage:

– Restrictions: Check limits, get remaining quota
– Custom Fields: CRUD operations, get field definitions
– Co-Authors: Invite, accept, decline, list collaborators
– Groups: Link/unlink, list group posts, feature posts
– Credits: Balance, transactions, purchase
– Series: Create, manage, reorder posts
– Analytics: Retrieve stats, export data
– Scheduling: Create, update, cancel schedules
– OpenAPI documentation
– Rate limiting per endpoint

Enables mobile apps and third-party integrations.