feat: Complete HTML-first architecture implementation with API integration

- Replace value field with html_content for direct HTML storage
- Add original_template field for style detection preservation
- Remove all markdown processing from injector (delete markdown.go)
- Fix critical content extraction/injection bugs in engine
- Add missing UpdateContent PUT handler for content persistence
- Fix API client field names and add updateContent() method
- Resolve content type validation (only allow text/link types)
- Add UUID-based ID generation to prevent collisions
- Complete first-pass processing workflow for unprocessed elements
- Verify end-to-end: Enhancement → Database → API → Editor → Persistence

All 37 files updated for HTML-first content management system.
Phase 3a implementation complete and production ready.

lib/src/core/insertr.js

@@ -134,24 +134,13 @@ export class InsertrCore {
     detectContentType(element) {
         const tag = element.tagName.toLowerCase();
         
-        if (element.classList.contains('insertr-group')) {
-            return 'group';
+        // Only return database-valid types: 'text' or 'link'
+        if (tag === 'a' || tag === 'button') {
+            return 'link';
         }
         
-        switch (tag) {
-            case 'h1': case 'h2': case 'h3': case 'h4': case 'h5': case 'h6':
-                return 'text';
-            case 'p':
-                return 'textarea';
-            case 'a': case 'button':
-                return 'link';
-            case 'div': case 'section':
-                return 'text';
-            case 'span':
-                return 'text';
-            default:
-                return 'text';
-        }
+        // All other elements are text content
+        return 'text';
     }
     
     // Get all elements with their metadata, including group elements