fix: add vendor page creation endpoint and update JS logic

- Add POST /admin/content-pages/vendor endpoint for vendor overrides
- Update JS to use /vendor or /platform endpoint based on vendor selection
- Platform endpoint forces vendor_id=None, vendor endpoint requires vendor_id

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/admin/js/content-page-edit.js

@@ -163,10 +163,13 @@ function contentPageEditor(pageId) {
                     this.successMessage = 'Page updated successfully!';
                     contentPageEditLog.info('Page updated');
                 } else {
-                    // Create new page
-                    response = await apiClient.post('/admin/content-pages/platform', payload);
+                    // Create new page - use vendor or platform endpoint based on selection
+                    const endpoint = this.form.vendor_id
+                        ? '/admin/content-pages/vendor'
+                        : '/admin/content-pages/platform';
+                    response = await apiClient.post(endpoint, payload);
                     this.successMessage = 'Page created successfully!';
-                    contentPageEditLog.info('Page created');
+                    contentPageEditLog.info('Page created', { endpoint, vendor_id: this.form.vendor_id });
 
                     // Redirect to edit page after creation
                     const pageData = response.data || response;