Implement many-to-many TODO user assignments

- Create todo_users junction table for many-to-many relationship
- Add migration to create todo_users table and migrate existing data
- Update validators to accept assignedUserIds array instead of assignedTo
- Update todo service to handle multiple user assignments
- Fetch and return assigned users with each TODO

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

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

src/controllers/todo.controller.js

@@ -100,13 +100,14 @@ export const getTodoWithRelations = async (req, res) => {
 /**
  * Create new todo
  * POST /api/todos
- * Body: { title, description, projectId, companyId, assignedTo, status, priority, dueDate }
+ * Body: { title, description, projectId, companyId, assignedUserIds, status, priority, dueDate }
  */
 export const createTodo = async (req, res) => {
   try {
     const userId = req.userId;
     const data = req.body;
 
+    console.log('Backend received todo data:', data);
     const todo = await todoService.createTodo(userId, data);
 
     res.status(201).json({
@@ -123,13 +124,14 @@ export const createTodo = async (req, res) => {
 /**
  * Update todo
  * PATCH /api/todos/:todoId
- * Body: { title, description, projectId, companyId, assignedTo, status, priority, dueDate }
+ * Body: { title, description, projectId, companyId, assignedUserIds, status, priority, dueDate }
  */
 export const updateTodo = async (req, res) => {
   try {
     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({