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

internal/api/models.go

@@ -50,12 +50,6 @@ type CreateContentRequest struct {
 	CreatedBy      string          `json:"created_by,omitempty"`
 }
 
-type UpdateContentRequest struct {
-	Value     string `json:"value"`
-	Type      string `json:"type,omitempty"`
-	UpdatedBy string `json:"updated_by,omitempty"`
-}
-
 type RollbackContentRequest struct {
 	VersionID    int64  `json:"version_id"`
 	RolledBackBy string `json:"rolled_back_by,omitempty"`