Consolidate type definitions and fix API contract

- Move all ContentItem, ContentClient, ContentResponse types to engine/types.go as single source of truth
- Remove duplicate type definitions from content/types.go
- Update all imports across codebase to use engine types
- Enhance engine to extract existing data-content-id from HTML markup
- Simplify frontend to always send html_markup, let server handle ID extraction/generation
- Fix contentId reference errors in frontend error handling
- Add getAttribute helper method to engine for ID extraction
- Add GetAllContent method to engine.DatabaseClient
- Update enhancer to use engine.ContentClient interface
- All builds and API endpoints verified working

This resolves the 400 Bad Request errors and creates a unified architecture where the server is the single source of truth for all ID generation and content type management.

internal/engine/engine.go

@@ -37,20 +37,33 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 	processedElements := make([]ProcessedElement, len(elements))
 
 	for i, elem := range elements {
-		// Generate ID using the same algorithm as the parser
-		id := e.idGenerator.Generate(elem.Node, input.FilePath)
+		// Check if element already has a data-content-id
+		existingID := e.getAttribute(elem.Node, "data-content-id")
+		var id string
+		var wasGenerated bool
+
+		if existingID != "" {
+			// Use existing ID from enhanced element
+			id = existingID
+			wasGenerated = false
+		} else {
+			// Generate new ID for unprocessed element
+			id = e.idGenerator.Generate(elem.Node, input.FilePath)
+			wasGenerated = true
+		}
+
 		generatedIDs[fmt.Sprintf("element_%d", i)] = id
 
 		processedElements[i] = ProcessedElement{
 			Node:      elem.Node,
 			ID:        id,
 			Type:      elem.Type,
-			Generated: true,
+			Generated: wasGenerated,
 			Tag:       elem.Node.Data,
 			Classes:   GetClasses(elem.Node),
 		}
 
-		// Add content attributes to the node
+		// Add/update content attributes to the node
 		e.addContentAttributes(elem.Node, id, elem.Type)
 	}
 
@@ -133,6 +146,16 @@ func (e *ContentEngine) addContentAttributes(node *html.Node, contentID, content
 	e.setAttribute(node, "data-content-type", contentType)
 }
 
+// getAttribute gets an attribute value from an HTML node
+func (e *ContentEngine) getAttribute(node *html.Node, key string) string {
+	for _, attr := range node.Attr {
+		if attr.Key == key {
+			return attr.Val
+		}
+	}
+	return ""
+}
+
 // setAttribute sets an attribute on an HTML node
 func (e *ContentEngine) setAttribute(node *html.Node, key, value string) {
 	// Remove existing attribute if it exists