feat: implement structural IDs with database-first existence checking

- Remove content preview from ID generation for stable structural IDs
- Implement database-first approach to check content existence before creation
- Add enhanced DOM path, semantic context, and precise sibling indexing
- Replace HTML attribute checking with reliable database lookups
- Add collision handling with increment counters for similar elements

Fixes:
✅ UNIQUE constraint errors eliminated (multiple enhancement runs work)
✅ Structural stability (same element keeps same ID regardless of content changes)
✅ Database-driven workflow (single source of truth for content existence)
✅ Enhanced collision resistance with detailed structural differentiation

Results: No more 'Failed to store content' errors, stable enhance workflow.

internal/engine/engine.go

@@ -60,20 +60,12 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 	processedElements := make([]ProcessedElement, len(elements))
 
 	for i, elem := range elements {
-		// Check if element already has a data-content-id
-		existingID := e.getAttribute(elem.Node, "data-content-id")
-		var id string
-		var wasGenerated bool
+		// Generate structural ID (always deterministic)
+		id := e.idGenerator.Generate(elem.Node, input.FilePath)
 
-		if existingID != "" {
-			// Use existing ID from enhanced element
-			id = existingID
-			wasGenerated = false
-		} else {
-			// Generate new ID for unprocessed element
-			id = e.idGenerator.Generate(elem.Node, input.FilePath)
-			wasGenerated = true
-		}
+		// Database-first approach: Check if content already exists
+		existingContent, err := e.client.GetContent(input.SiteID, id)
+		contentExists := (err == nil && existingContent != nil)
 
 		generatedIDs[fmt.Sprintf("element_%d", i)] = id
 
@@ -81,7 +73,7 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 			Node:      elem.Node,
 			ID:        id,
 			Type:      elem.Type,
-			Generated: wasGenerated,
+			Generated: !contentExists, // Mark as generated only if new to database
 			Tag:       elem.Node.Data,
 			Classes:   GetClasses(elem.Node),
 		}
@@ -89,8 +81,8 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 		// Add/update content attributes to the node
 		e.addContentAttributes(elem.Node, id, elem.Type)
 
-		// Store content and template for newly discovered elements (first-pass)
-		if wasGenerated && (input.Mode == Enhancement || input.Mode == ContentInjection) {
+		// Store content only for truly new elements (database-first check)
+		if !contentExists && (input.Mode == Enhancement || input.Mode == ContentInjection) {
 			// Extract content and template from the unprocessed element
 			htmlContent := e.extractHTMLContent(elem.Node)
 			originalTemplate := e.extractOriginalTemplate(elem.Node)
@@ -100,6 +92,8 @@ func (e *ContentEngine) ProcessContent(input ContentInput) (*ContentResult, erro
 			if err != nil {
 				// Log error but don't fail the enhancement - content just won't be stored
 				fmt.Printf("⚠️ Failed to store content for %s: %v\n", id, err)
+			} else {
+				fmt.Printf("✅ Created new content: %s (%s)\n", id, elem.Type)
 			}
 		}
 	}