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/database_client.go

@@ -110,3 +110,47 @@ func (c *DatabaseClient) GetBulkContent(siteID string, contentIDs []string) (map
 		return nil, fmt.Errorf("unsupported database type: %s", c.database.GetDBType())
 	}
 }
+
+// GetAllContent retrieves all content items for a site
+func (c *DatabaseClient) GetAllContent(siteID string) (map[string]ContentItem, error) {
+	switch c.database.GetDBType() {
+	case "sqlite3":
+		contents, err := c.database.GetSQLiteQueries().GetAllContent(context.Background(), siteID)
+		if err != nil {
+			return nil, err
+		}
+
+		items := make(map[string]ContentItem)
+		for _, content := range contents {
+			items[content.ID] = ContentItem{
+				ID:           content.ID,
+				SiteID:       content.SiteID,
+				Value:        content.Value,
+				Type:         content.Type,
+				LastEditedBy: content.LastEditedBy,
+			}
+		}
+		return items, nil
+
+	case "postgresql":
+		contents, err := c.database.GetPostgreSQLQueries().GetAllContent(context.Background(), siteID)
+		if err != nil {
+			return nil, err
+		}
+
+		items := make(map[string]ContentItem)
+		for _, content := range contents {
+			items[content.ID] = ContentItem{
+				ID:           content.ID,
+				SiteID:       content.SiteID,
+				Value:        content.Value,
+				Type:         content.Type,
+				LastEditedBy: content.LastEditedBy,
+			}
+		}
+		return items, nil
+
+	default:
+		return nil, fmt.Errorf("unsupported database type: %s", c.database.GetDBType())
+	}
+}