Add GET /categories and GET /tags to list available product categories and tags

Description: As a developer integrating with the API, I want dedicated endpoints GET /categories and GET /tags to retrieve all available categories and tags for a product so that I can dynamically populate options, validate inputs, and avoid hardcoding IDs. Problem: Currently, the API allows setting a category when creating a post and tags when updating a post, but there is no API method to list available categories or tags. Developers must manually look up and hardcode IDs from the dashboard and cannot detect when categories or tags change. This makes automations fragile and error-prone. Benefit: Providing GET /categories and GET /tags enables dynamic discovery of valid categories and tags, reducing manual maintenance and preventing mismatches. Integrations can validate user input, adapt automatically to taxonomy changes, and improve reliability of automated workflows, leading to fewer failures and support issues.