Refactor: code quality improvements

- Extract admin.service.js from admin.controller.js (proper layering)
- Remove console.log statements from todo.controller.js
- Fix inconsistent error handling in auth.controller.js (return next)
- Remove logger.debug calls from contact.controller.js
- Add transaction management to contact.service.js addContact()

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

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

src/controllers/todo.controller.js

@@ -110,7 +110,6 @@ export const createTodo = async (req, res, next) => {
     const userId = req.userId;
     const data = req.body;
 
-    console.log('Backend received todo data:', data);
     const todo = await todoService.createTodo(userId, data);
 
     // Log audit event
@@ -136,7 +135,6 @@ export const updateTodo = async (req, res, next) => {
     const { todoId } = req.params;
     const data = req.body;
 
-    console.log('Backend received update data:', data);
     const todo = await todoService.updateTodo(todoId, data);
 
     res.status(200).json({