Implement atomic collection item creation API with unified content engine approach

Updates collection creation to use database-first atomic operations for reliable collection item management. Replaces manual database calls with unified content engine methods that handle content extraction, storage, and structural template generation consistently.

Key changes:
- Replace manual database operations in CreateCollectionItem handler with DatabaseClient.CreateCollectionItemAtomic()
- Implement unified content engine approach for API-based collection item creation
- Add atomic collection item creation methods across all content clients
- Enhance reconstruction to use stored structural templates with content ID hydration
- Add comprehensive collection management API methods in JavaScript client
- Implement collection manager UI with create, delete, and reorder functionality

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/engine/types.go

@@ -41,6 +41,14 @@ type ProcessedElement struct {
 	Classes   []string   // Element CSS classes
 }
 
+// ContentEntry represents a content item to be created for collection templates
+type ContentEntry struct {
+	ID          string
+	SiteID      string
+	HTMLContent string
+	Template    string
+}
+
 // ContentClient interface for accessing content data
 // This will be implemented by database clients, HTTP clients, and mock clients
 type ContentClient interface {
@@ -56,6 +64,7 @@ type ContentClient interface {
 	GetCollectionTemplates(siteID, collectionID string) ([]CollectionTemplateItem, error)
 	CreateCollectionTemplate(siteID, collectionID, name, htmlTemplate string, isDefault bool) (*CollectionTemplateItem, error)
 	CreateCollectionItem(siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error)
+	CreateCollectionItemAtomic(siteID, collectionID string, templateID int, lastEditedBy string) (*CollectionItemWithTemplate, error)
 }
 
 // ContentItem represents a piece of content from the database