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.

lib/src/core/insertr.js

@@ -106,6 +106,11 @@ export class InsertrCore {
     getElementMetadata(element) {
         const existingId = element.getAttribute('data-content-id');
         
+        // Ensure element has insertr class for server processing
+        if (!element.classList.contains('insertr')) {
+            element.classList.add('insertr');
+        }
+        
         // Send HTML markup to server for unified ID generation
         return {
             contentId: existingId,  // null if new content, existing ID if updating