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/site_manager.go

@@ -7,6 +7,8 @@ import (
 	"path/filepath"
 	"sync"
 	"time"
+
+	"github.com/insertr/insertr/internal/engine"
 )
 
 // SiteConfig represents configuration for a registered site
@@ -28,7 +30,7 @@ type SiteManager struct {
 }
 
 // NewSiteManager creates a new site manager
-func NewSiteManager(contentClient ContentClient, backupDir string, devMode bool) *SiteManager {
+func NewSiteManager(contentClient engine.ContentClient, backupDir string, devMode bool) *SiteManager {
 	if backupDir == "" {
 		backupDir = "./insertr-backups"
 	}