fix: improve error handling in categories service

- Slug collision checks now properly propagate real DB errors instead of
  silently ignoring them
- GetCategory now verifies site ownership, matching the pattern used by
  UpdateCategory and DeleteCategory

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/categories/service.go

@@ -61,7 +61,19 @@ func (s *Service) ListCategoriesWithCounts(ctx context.Context) ([]models.Catego
 }
 
 func (s *Service) GetCategory(ctx context.Context, id int64) (*models.Category, error) {
-	return s.db.SelectCategory(ctx, id)
+	site, ok := models.GetSite(ctx)
+	if !ok {
+		return nil, models.SiteRequiredError
+	}
+
+	cat, err := s.db.SelectCategory(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	if cat.SiteID != site.ID {
+		return nil, models.NotFoundError
+	}
+	return cat, nil
 }
 
 func (s *Service) CreateCategory(ctx context.Context, params CreateCategoryParams) (*models.Category, error) {
@@ -79,6 +91,8 @@ func (s *Service) CreateCategory(ctx context.Context, params CreateCategoryParam
 	// Check for slug collision
 	if _, err := s.db.SelectCategoryBySlugAndSite(ctx, site.ID, slug); err == nil {
 		return nil, models.SlugConflictError
+	} else if !db.ErrorIsNoRows(err) {
+		return nil, err
 	}
 
 	cat := &models.Category{
@@ -124,6 +138,8 @@ func (s *Service) UpdateCategory(ctx context.Context, id int64, params CreateCat
 	// Check slug collision (exclude self)
 	if existing, err := s.db.SelectCategoryBySlugAndSite(ctx, site.ID, slug); err == nil && existing.ID != cat.ID {
 		return nil, models.SlugConflictError
+	} else if err != nil && !db.ErrorIsNoRows(err) {
+		return nil, err
 	}
 
 	cat.Name = params.Name