feat: add image management functionality

- Implemented image folder repository with CRUD operations.
- Added image reference repository for managing image references to articles.
- Created image repository for handling image assets, including listing, inserting, updating, and deleting images.
- Introduced image usage repository to track storage usage and quotas for tenants.
- Added SQL queries for image assets, folders, references, and usage.
- Developed image handler for HTTP endpoints to manage images and folders.
- Created database migration scripts for image-related tables and structures.

server/migrations/20260416113000_add_image_asset_content_hash.up.sql

@@ -0,0 +1,8 @@
+ALTER TABLE image_assets
+ADD COLUMN content_hash VARCHAR(64);
+
+CREATE INDEX idx_image_assets_tenant_content_hash_active
+ON image_assets (tenant_id, content_hash)
+WHERE content_hash IS NOT NULL
+  AND status = 'active'
+  AND deleted_at IS NULL;