feat: implement unified content engine to eliminate ID generation inconsistencies

- Create internal/engine module as single source of truth for content processing
- Consolidate 4 separate ID generation systems into one unified engine
- Update API handlers to use engine for consistent server-side ID generation
- Remove frontend client-side ID generation, delegate to server engine
- Ensure identical HTML markup + file path produces identical content IDs
- Resolve content persistence failures caused by ID fragmentation between manual editing and enhancement processes

internal/api/models.go

@@ -42,12 +42,12 @@ type ElementContext struct {
 
 // Request models
 type CreateContentRequest struct {
-	ID             string          `json:"id,omitempty"`              // For enhanced sites
-	ElementContext *ElementContext `json:"element_context,omitempty"` // For non-enhanced sites
-	SiteID         string          `json:"site_id,omitempty"`
-	Value          string          `json:"value"`
-	Type           string          `json:"type"`
-	CreatedBy      string          `json:"created_by,omitempty"`
+	HTMLMarkup string `json:"html_markup"`          // HTML markup of the element
+	FilePath   string `json:"file_path"`            // File path for consistent ID generation
+	Value      string `json:"value"`                // Content value
+	Type       string `json:"type"`                 // Content type
+	SiteID     string `json:"site_id,omitempty"`    // Site identifier
+	CreatedBy  string `json:"created_by,omitempty"` // User who created the content
 }
 
 type RollbackContentRequest struct {