fix(collections): add authentication check to PreviewCollection handler

The PreviewCollection endpoint was missing authentication verification,
allowing unauthenticated access to the preview functionality. Added
check for user in context, returning 401 Unauthorized if missing.

internal/handlers/collections.go

@@ -827,6 +827,11 @@ func (h *CollectionHandler) HandleBulkAddBooks(c echo.Context) error {
 }
 
 func (h *CollectionHandler) PreviewCollection(c echo.Context) error {
+	user := c.Get("user")
+	if user == nil {
+		return c.JSON(http.StatusUnauthorized, map[string]string{"error": "unauthorized"})
+	}
+
 	var req struct {
 		LibraryID     string          `json:"library_id"`
 		Rules         []services.Rule `json:"rules"`