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/engine/types.go

@@ -0,0 +1,59 @@
+package engine
+
+import (
+	"golang.org/x/net/html"
+)
+
+// ProcessMode defines how the engine should process content
+type ProcessMode int
+
+const (
+	// Enhancement mode: Parse + Generate IDs + Inject content + Add editor assets
+	Enhancement ProcessMode = iota
+	// IDGeneration mode: Parse + Generate IDs only (for API)
+	IDGeneration
+	// ContentInjection mode: Parse + Generate IDs + Inject content only
+	ContentInjection
+)
+
+// ContentInput represents input to the content engine
+type ContentInput struct {
+	HTML     []byte      // Raw HTML or markup
+	FilePath string      // File context (e.g., "index.html")
+	SiteID   string      // Site identifier
+	Mode     ProcessMode // Processing mode
+}
+
+// ContentResult represents the result of content processing
+type ContentResult struct {
+	Document     *html.Node         // Processed HTML document
+	Elements     []ProcessedElement // All processed elements
+	GeneratedIDs map[string]string  // Map of element positions to generated IDs
+}
+
+// ProcessedElement represents an element that has been processed
+type ProcessedElement struct {
+	Node      *html.Node // HTML node
+	ID        string     // Generated content ID
+	Type      string     // Content type (text, markdown, link)
+	Content   string     // Injected content (if any)
+	Generated bool       // Whether ID was generated (vs existing)
+	Tag       string     // Element tag name
+	Classes   []string   // Element CSS classes
+}
+
+// ContentClient interface for accessing content data
+// This will be implemented by database clients
+type ContentClient interface {
+	GetContent(siteID, contentID string) (*ContentItem, error)
+	GetBulkContent(siteID string, contentIDs []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
+}