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/mock.go

@@ -2,17 +2,19 @@ package content
 
 import (
 	"time"
+
+	"github.com/insertr/insertr/internal/engine"
 )
 
 // MockClient implements ContentClient with mock data for development
 type MockClient struct {
-	data map[string]ContentItem
+	data map[string]engine.ContentItem
 }
 
 // NewMockClient creates a new mock content client with sample data
 func NewMockClient() *MockClient {
 	// Generate realistic mock content based on actual generated IDs
-	data := map[string]ContentItem{
+	data := map[string]engine.ContentItem{
 		// Navigation (index.html has collision suffix)
 		"navbar-logo-2b10ad": {
 			ID:        "navbar-logo-2b10ad",
@@ -98,7 +100,7 @@ func NewMockClient() *MockClient {
 }
 
 // GetContent fetches a single content item by ID
-func (m *MockClient) GetContent(siteID, contentID string) (*ContentItem, error) {
+func (m *MockClient) GetContent(siteID, contentID string) (*engine.ContentItem, error) {
 	if item, exists := m.data[contentID]; exists && item.SiteID == siteID {
 		return &item, nil
 	}
@@ -108,8 +110,8 @@ func (m *MockClient) GetContent(siteID, contentID string) (*ContentItem, error)
 }
 
 // GetBulkContent fetches multiple content items by IDs
-func (m *MockClient) GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error) {
-	result := make(map[string]ContentItem)
+func (m *MockClient) GetBulkContent(siteID string, contentIDs []string) (map[string]engine.ContentItem, error) {
+	result := make(map[string]engine.ContentItem)
 
 	for _, id := range contentIDs {
 		item, err := m.GetContent(siteID, id)
@@ -125,8 +127,8 @@ func (m *MockClient) GetBulkContent(siteID string, contentIDs []string) (map[str
 }
 
 // GetAllContent fetches all content for a site
-func (m *MockClient) GetAllContent(siteID string) (map[string]ContentItem, error) {
-	result := make(map[string]ContentItem)
+func (m *MockClient) GetAllContent(siteID string) (map[string]engine.ContentItem, error) {
+	result := make(map[string]engine.ContentItem)
 
 	for _, item := range m.data {
 		if item.SiteID == siteID {