Add asset library for centralized image and file management

[sagnik11]

TL;DR

Implements a centralized asset library that allows users to upload images and files once, then browse and reuse them across campaigns. Users can now select assets from the library directly in the email editor via a "Choose from library" picker, eliminating the need for per-campaign image uploads.

What changed?

• Database schema: Added assets table to store metadata for uploaded images and files (fileId, name, mimeType, size, s3Key, dimensions, timestamps)
• Image upload endpoint: Modified /api/internal/images to persist asset metadata to the database after file upload
• Asset picker component: Created AssetPicker component for browsing and selecting assets from the library
• Image block editor: Added "Choose from library" button to the ImageBlock editor that opens the asset picker and sets the selected asset URL
• Navigation: Added "Assets" menu item to the main sidebar for accessing the asset library
• Documentation: Updated CHANGELOG.md and README.md to document the asset library feature, and marked the feature as complete in ROADMAP.md