Add API Endpoints for Categories & Tags

Description: Add dedicated API endpoints (GET /categories and GET /tags) to retrieve all available categories and tags for a product. Problem: The current API allows setting a category (type) when creating a post and tags when updating a post, but there is no way to programmatically retrieve the available categories and tags. This means developers must manually look up IDs in the dashboard and hardcode them, with no way to detect when categories or tags are added, renamed, or removed. This makes any automation built on top of the API fragile and error-prone. Benefit: These endpoints would enable reliable automation and integrations by allowing developers to dynamically fetch and validate categories and tags. This eliminates the need for manual synchronization, reduces the risk of using outdated or incorrect IDs, and makes the API significantly more suitable for building robust third-party integrations.