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/api/models.go

@@ -4,23 +4,25 @@ import "time"
 
 // API request/response models
 type ContentItem struct {
-	ID           string    `json:"id"`
-	SiteID       string    `json:"site_id"`
-	Value        string    `json:"value"`
-	Type         string    `json:"type"`
-	CreatedAt    time.Time `json:"created_at"`
-	UpdatedAt    time.Time `json:"updated_at"`
-	LastEditedBy string    `json:"last_edited_by"`
+	ID               string    `json:"id"`
+	SiteID           string    `json:"site_id"`
+	HTMLContent      string    `json:"html_content"`
+	OriginalTemplate string    `json:"original_template"`
+	Type             string    `json:"type"`
+	CreatedAt        time.Time `json:"created_at"`
+	UpdatedAt        time.Time `json:"updated_at"`
+	LastEditedBy     string    `json:"last_edited_by"`
 }
 
 type ContentVersion struct {
-	VersionID int64     `json:"version_id"`
-	ContentID string    `json:"content_id"`
-	SiteID    string    `json:"site_id"`
-	Value     string    `json:"value"`
-	Type      string    `json:"type"`
-	CreatedAt time.Time `json:"created_at"`
-	CreatedBy string    `json:"created_by"`
+	VersionID        int64     `json:"version_id"`
+	ContentID        string    `json:"content_id"`
+	SiteID           string    `json:"site_id"`
+	HTMLContent      string    `json:"html_content"`
+	OriginalTemplate string    `json:"original_template"`
+	Type             string    `json:"type"`
+	CreatedAt        time.Time `json:"created_at"`
+	CreatedBy        string    `json:"created_by"`
 }
 
 type ContentResponse struct {
@@ -42,12 +44,13 @@ type ElementContext struct {
 
 // Request models
 type CreateContentRequest struct {
-	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
+	HTMLMarkup       string `json:"html_markup"`          // HTML markup of the element
+	FilePath         string `json:"file_path"`            // File path for consistent ID generation
+	HTMLContent      string `json:"html_content"`         // HTML content value
+	OriginalTemplate string `json:"original_template"`    // Original template markup
+	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 {