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/database_client.go

@@ -516,3 +516,36 @@ func (c *DatabaseClient) CreateCollectionItem(siteID, collectionID, itemID strin
 		return nil, fmt.Errorf("unsupported database type: %s", c.database.GetDBType())
 	}
 }
+
+// CreateCollectionItemAtomic creates a collection item with all its content entries atomically
+func (c *DatabaseClient) CreateCollectionItemAtomic(
+	siteID, collectionID string,
+	templateID int,
+	lastEditedBy string,
+) (*CollectionItemWithTemplate, error) {
+	// Get template HTML for processing
+	templates, err := c.GetCollectionTemplates(siteID, collectionID)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get templates: %w", err)
+	}
+
+	var templateHTML string
+	for _, template := range templates {
+		if template.TemplateID == templateID {
+			templateHTML = template.HTMLTemplate
+			break
+		}
+	}
+
+	if templateHTML == "" {
+		return nil, fmt.Errorf("template %d not found", templateID)
+	}
+
+	// Use unified engine approach (no more TemplateProcessor)
+	engine := NewContentEngine(c)
+
+	// Create collection item using unified engine method
+	return engine.CreateCollectionItemFromTemplate(
+		siteID, collectionID, templateID, templateHTML, lastEditedBy,
+	)
+}