feat: implement single POST upsert API with automatic ID generation

- Replace separate POST/PUT endpoints with unified POST upsert
- Add automatic content ID generation from element context when no ID provided
- Implement version history preservation before content updates
- Add element context support for backend ID generation
- Update frontend to use single endpoint for all content operations
- Enhanced demo site with latest database content including proper content IDs

lib/src/core/insertr.js

@@ -106,24 +106,14 @@ export class InsertrCore {
     getElementMetadata(element) {
         const existingId = element.getAttribute('data-content-id');
         
-        if (existingId) {
-            // Enhanced site - use existing ID
-            return {
-                contentId: existingId,
-                contentType: element.getAttribute('data-content-type') || this.detectContentType(element),
-                element: element,
-                hasExistingId: true
-            };
-        } else {
-            // Non-enhanced site - prepare context for backend ID generation
-            return {
-                contentId: null,  // Backend will generate
-                contentType: this.detectContentType(element),
-                element: element,
-                elementContext: this.extractElementContext(element),
-                hasExistingId: false
-            };
-        }
+        // Always provide both existing ID (if any) and element context
+        // Backend will use existing ID if provided, or generate new one from context
+        return {
+            contentId: existingId,  // null if new content, existing ID if updating
+            contentType: element.getAttribute('data-content-type') || this.detectContentType(element),
+            element: element,
+            elementContext: this.extractElementContext(element)
+        };
     }
     
     // Extract element context for backend ID generation