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/api-client.js

@@ -28,36 +28,7 @@ export class ApiClient {
         }
     }
     
-    async updateContent(contentId, content) {
-        try {
-            const response = await fetch(`${this.baseUrl}/${contentId}?site_id=${this.siteId}`, {
-                method: 'PUT',
-                headers: {
-                    'Content-Type': 'application/json',
-                    'Authorization': `Bearer ${this.getAuthToken()}`
-                },
-                body: JSON.stringify({ value: content })
-            });
-            
-            if (response.ok) {
-                console.log(`✅ Content updated: ${contentId}`);
-                return true;
-            } else {
-                console.warn(`⚠️ Update failed (${response.status}): ${contentId}`);
-                return false;
-            }
-        } catch (error) {
-            // Provide helpful error message for common development issues
-            if (error.name === 'TypeError' && error.message.includes('fetch')) {
-                console.warn(`🔌 API Server not reachable at ${this.baseUrl}`);
-                console.warn('💡 Start full-stack development: just dev');
-            } else {
-                console.error('Failed to update content:', contentId, error);
-            }
-            return false;
-        }
-    }
-    
+
     async createContent(contentId, content, type, elementContext = null) {
         try {
             const payload = {

lib/src/core/editor.js

@@ -102,27 +102,22 @@ export class InsertrEditor {
                 contentValue = formData.text || formData;
             }
             
-            if (meta.hasExistingId) {
-                // Enhanced site - update existing content
-                const updateSuccess = await this.apiClient.updateContent(meta.contentId, contentValue);
-                if (!updateSuccess) {
-                    console.error('❌ Failed to update content:', meta.contentId);
-                } else {
-                    console.log(`✅ Content updated:`, meta.contentId, contentValue);
-                }
+            // Universal upsert - works for both new and existing content
+            const contentType = this.determineContentType(meta.element);
+            const result = await this.apiClient.createContent(
+                meta.contentId, // Use existing ID if available, null if new
+                contentValue, 
+                contentType, 
+                meta.elementContext
+            );
+            
+            if (result) {
+                // Store the backend-generated/confirmed ID in the element
+                meta.element.setAttribute('data-content-id', result.id);
+                meta.element.setAttribute('data-content-type', result.type);
+                console.log(`✅ Content saved: ${result.id}`, contentValue);
             } else {
-                // Non-enhanced site - create with backend ID generation
-                const contentType = this.determineContentType(meta.element);
-                const result = await this.apiClient.createContent(null, contentValue, contentType, meta.elementContext);
-                
-                if (result) {
-                    // Store the backend-generated ID in the element
-                    meta.element.setAttribute('data-content-id', result.id);
-                    meta.element.setAttribute('data-content-type', result.type);
-                    console.log(`✅ Content created with backend ID: ${result.id}`, contentValue);
-                } else {
-                    console.error('❌ Failed to save content to server');
-                }
+                console.error('❌ Failed to save content to server');
             }
             
             // Close form

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