Consolidate type definitions and fix API contract

- Move all ContentItem, ContentClient, ContentResponse types to engine/types.go as single source of truth
- Remove duplicate type definitions from content/types.go
- Update all imports across codebase to use engine types
- Enhance engine to extract existing data-content-id from HTML markup
- Simplify frontend to always send html_markup, let server handle ID extraction/generation
- Fix contentId reference errors in frontend error handling
- Add getAttribute helper method to engine for ID extraction
- Add GetAllContent method to engine.DatabaseClient
- Update enhancer to use engine.ContentClient interface
- All builds and API endpoints verified working

This resolves the 400 Bad Request errors and creates a unified architecture where the server is the single source of truth for all ID generation and content type management.

internal/engine/types.go

@@ -43,17 +43,25 @@ type ProcessedElement struct {
 }
 
 // ContentClient interface for accessing content data
-// This will be implemented by database clients
+// This will be implemented by database clients, HTTP clients, and mock clients
 type ContentClient interface {
 	GetContent(siteID, contentID string) (*ContentItem, error)
 	GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error)
+	GetAllContent(siteID string) (map[string]ContentItem, error)
 }
 
 // ContentItem represents a piece of content from the database
 type ContentItem struct {
-	ID           string
-	SiteID       string
-	Value        string
-	Type         string
-	LastEditedBy string
+	ID           string `json:"id"`
+	SiteID       string `json:"site_id"`
+	Value        string `json:"value"`
+	Type         string `json:"type"`
+	UpdatedAt    string `json:"updated_at"`
+	LastEditedBy string `json:"last_edited_by,omitempty"`
+}
+
+// ContentResponse represents the API response structure
+type ContentResponse struct {
+	Content []ContentItem `json:"content"`
+	Error   string        `json:"error,omitempty"`
 }