add feed generators for Google Merchant, Amazon Seller, Yandex Market, and Yandex Products. Includes a shared base class (`BaseFeedGenerator`) for common functionality, such as product retrieval and validation. Each generator supports format-specific output and handles platform-specific requirements.
This change simplifies the creation of product feeds for multiple marketplaces.
Introduce `EmailImage`, `EmailTemplate`, and `EmailCampaign` models for enhanced email management, including campaign tracking and one-click unsubscribe tokens. Added multilingual fields for email templates and improved accessibility with image alt text. Also adapted `Post` content fields to support translations.
