Implement atomic collection item creation API with unified content engine approach

Updates collection creation to use database-first atomic operations for reliable collection item management. Replaces manual database calls with unified content engine methods that handle content extraction, storage, and structural template generation consistently.

Key changes:
- Replace manual database operations in CreateCollectionItem handler with DatabaseClient.CreateCollectionItemAtomic()
- Implement unified content engine approach for API-based collection item creation
- Add atomic collection item creation methods across all content clients
- Enhance reconstruction to use stored structural templates with content ID hydration
- Add comprehensive collection management API methods in JavaScript client
- Implement collection manager UI with create, delete, and reorder functionality

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/api/handlers.go

@@ -1004,45 +1004,37 @@ func (h *ContentHandler) CreateCollectionItem(w http.ResponseWriter, r *http.Req
 	if req.CollectionID == "" {
 		req.CollectionID = collectionID
 	}
-
-	// Generate item ID
-	itemID := fmt.Sprintf("%s-item-%d", collectionID, time.Now().Unix())
-
-	var createdItem interface{}
-	var err error
-
-	switch h.database.GetDBType() {
-	case "sqlite3":
-		createdItem, err = h.database.GetSQLiteQueries().CreateCollectionItem(context.Background(), sqlite.CreateCollectionItemParams{
-			ItemID:       itemID,
-			CollectionID: req.CollectionID,
-			SiteID:       req.SiteID,
-			TemplateID:   int64(req.TemplateID),
-			HtmlContent:  req.HTMLContent,
-			Position:     int64(req.Position),
-			LastEditedBy: req.CreatedBy,
-		})
-	case "postgresql":
-		createdItem, err = h.database.GetPostgreSQLQueries().CreateCollectionItem(context.Background(), postgresql.CreateCollectionItemParams{
-			ItemID:       itemID,
-			CollectionID: req.CollectionID,
-			SiteID:       req.SiteID,
-			TemplateID:   int32(req.TemplateID),
-			HtmlContent:  req.HTMLContent,
-			Position:     int32(req.Position),
-			LastEditedBy: req.CreatedBy,
-		})
-	default:
-		http.Error(w, "Unsupported database type", http.StatusInternalServerError)
-		return
+	if req.TemplateID == 0 {
+		req.TemplateID = 1 // Default to first template
 	}
 
+	// Create database client for atomic operations
+	dbClient := engine.NewDatabaseClient(h.database)
+
+	// Use atomic collection item creation
+	createdItem, err := dbClient.CreateCollectionItemAtomic(
+		req.SiteID,
+		req.CollectionID,
+		req.TemplateID,
+		req.CreatedBy,
+	)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("Failed to create collection item: %v", err), http.StatusInternalServerError)
 		return
 	}
 
-	apiItem := h.convertToAPICollectionItem(createdItem)
+	// Convert to API response format
+	apiItem := CollectionItemData{
+		ItemID:       createdItem.ItemID,
+		CollectionID: createdItem.CollectionID,
+		SiteID:       createdItem.SiteID,
+		TemplateID:   createdItem.TemplateID,
+		HTMLContent:  createdItem.HTMLContent,
+		Position:     createdItem.Position,
+		CreatedAt:    time.Now(),
+		UpdatedAt:    time.Now(),
+		LastEditedBy: createdItem.LastEditedBy,
+	}
 
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusCreated)