fix(store): return ErrRecordNotFound when updating non-existent template

RowsAffected == 0 now returns gorm.ErrRecordNotFound so the handler can respond with 404 instead of silently returning 200.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/store/template_store.go

@@ -100,7 +100,13 @@ func (s *TemplateStore) Update(ctx context.Context, id int64, req *model.UpdateT
 	}
 
 	result := s.db.WithContext(ctx).Model(&model.JobTemplate{}).Where("id = ?", id).Updates(updates)
-	return result.Error
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return gorm.ErrRecordNotFound
+	}
+	return nil
 }
 
 // Delete removes a job template by ID. Idempotent — returns nil even if the row doesn't exist.