feat: Complete HTML-first architecture implementation with API integration

- Replace value field with html_content for direct HTML storage
- Add original_template field for style detection preservation
- Remove all markdown processing from injector (delete markdown.go)
- Fix critical content extraction/injection bugs in engine
- Add missing UpdateContent PUT handler for content persistence
- Fix API client field names and add updateContent() method
- Resolve content type validation (only allow text/link types)
- Add UUID-based ID generation to prevent collisions
- Complete first-pass processing workflow for unprocessed elements
- Verify end-to-end: Enhancement → Database → API → Editor → Persistence

All 37 files updated for HTML-first content management system.
Phase 3a implementation complete and production ready.

internal/engine/types.go

@@ -35,7 +35,7 @@ type ContentResult struct {
 type ProcessedElement struct {
 	Node      *html.Node // HTML node
 	ID        string     // Generated content ID
-	Type      string     // Content type (text, markdown, link)
+	Type      string     // Content type (text, link)
 	Content   string     // Injected content (if any)
 	Generated bool       // Whether ID was generated (vs existing)
 	Tag       string     // Element tag name
@@ -48,16 +48,18 @@ 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)
+	CreateContent(siteID, contentID, htmlContent, originalTemplate, contentType, lastEditedBy string) (*ContentItem, error)
 }
 
 // ContentItem represents a piece of content from the database
 type ContentItem struct {
-	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"`
+	ID               string `json:"id"`
+	SiteID           string `json:"site_id"`
+	HTMLContent      string `json:"html_content"`
+	OriginalTemplate string `json:"original_template"`
+	Type             string `json:"type"`
+	UpdatedAt        string `json:"updated_at"`
+	LastEditedBy     string `json:"last_edited_by,omitempty"`
 }
 
 // ContentResponse represents the API response structure