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.

cmd/enhance.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/insertr/insertr/internal/content"
 	"github.com/insertr/insertr/internal/db"
+	"github.com/insertr/insertr/internal/engine"
 )
 
 var enhanceCmd = &cobra.Command{
@@ -49,7 +50,7 @@ func runEnhance(cmd *cobra.Command, args []string) {
 	outputDir := viper.GetString("cli.output")
 
 	// Create content client
-	var client content.ContentClient
+	var client engine.ContentClient
 	if apiURL != "" {
 		fmt.Printf("🌐 Using content API: %s\n", apiURL)
 		client = content.NewHTTPClient(apiURL, apiKey)