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/content/enhancer.go

@@ -15,7 +15,7 @@ type Enhancer struct {
 }
 
 // NewEnhancer creates a new HTML enhancer using unified engine
-func NewEnhancer(client ContentClient, siteID string) *Enhancer {
+func NewEnhancer(client engine.ContentClient, siteID string) *Enhancer {
 	// Create database client for engine
 	var engineClient engine.ContentClient
 	if dbClient, ok := client.(*DatabaseClient); ok {