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/content/client.go

@@ -196,3 +196,7 @@ func (c *HTTPClient) GetCollectionTemplates(siteID, collectionID string) ([]engi
 func (c *HTTPClient) CreateCollectionItem(siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*engine.CollectionItemWithTemplate, error) {
 	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
 }
+
+func (c *HTTPClient) CreateCollectionItemAtomic(siteID, collectionID string, templateID int, lastEditedBy string) (*engine.CollectionItemWithTemplate, error) {
+	return nil, fmt.Errorf("collection operations not implemented in HTTPClient")
+}

internal/content/mock.go

@@ -182,3 +182,7 @@ func (m *MockClient) GetCollectionTemplates(siteID, collectionID string) ([]engi
 func (m *MockClient) CreateCollectionItem(siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*engine.CollectionItemWithTemplate, error) {
 	return nil, fmt.Errorf("collection operations not implemented in MockClient")
 }
+
+func (m *MockClient) CreateCollectionItemAtomic(siteID, collectionID string, templateID int, lastEditedBy string) (*engine.CollectionItemWithTemplate, error) {
+	return nil, fmt.Errorf("collection operations not implemented in MockClient")
+}