refactor(react-query): unify data layer and add weekly + image-upload support

[cto-new]

Summary

• Unifies data fetching through React Query across content, category, tag, analytics, weekly data, and image upload workflows.
• Introduces weekly-api.ts and backward-compatible shims; adds image upload hooks and exports for single/multiple uploads.
• Aligns MarkdownPreview/MarkdownEditor typings for content preview data and preserves compatibility with existing rendering paths.

Details

• Centralizes data fetching via a unified, pure-function service layer for content, category, tag, and analytics with backward-compatible wrappers.
• Adds weekly API module (weekly-api.ts) and adapts weekly queries to consume new API types; maintains compatibility with existing interfaces.
• Extends query surface with image upload capabilities: new hooks useImageUpload/useMultipleImageUpload, compress/thumbnail mutations; exports ImageUploadService and aliases for convenience.
• Improves type coverage for Markdown-based previews and aligns content preview data shape (title, content, content_type, category, tags).
• Updates planning/documentation to reflect the new architecture and provide guidance for future refactors.