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/db/schema.js

@@ -149,7 +149,6 @@ export const todos = pgTable('todos', {
   description: text('description'),
   projectId: uuid('project_id').references(() => projects.id, { onDelete: 'cascade' }), // todo môže patriť projektu
   companyId: uuid('company_id').references(() => companies.id, { onDelete: 'cascade' }), // alebo firme
-  assignedTo: uuid('assigned_to').references(() => users.id, { onDelete: 'set null' }), // komu je priradené
   status: todoStatusEnum('status').default('pending').notNull(),
   priority: todoPriorityEnum('priority').default('medium').notNull(),
   dueDate: timestamp('due_date'),
@@ -159,6 +158,17 @@ export const todos = pgTable('todos', {
   updatedAt: timestamp('updated_at').defaultNow().notNull(),
 });
 
+// Todo Users - many-to-many medzi todos a users (priradení používatelia)
+export const todoUsers = pgTable('todo_users', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  todoId: uuid('todo_id').references(() => todos.id, { onDelete: 'cascade' }).notNull(),
+  userId: uuid('user_id').references(() => users.id, { onDelete: 'cascade' }).notNull(),
+  assignedBy: uuid('assigned_by').references(() => users.id, { onDelete: 'set null' }), // kto pridal používateľa k todo
+  assignedAt: timestamp('assigned_at').defaultNow().notNull(),
+}, (table) => ({
+  todoUserUnique: unique('todo_user_unique').on(table.todoId, table.userId),
+}));
+
 // Notes table - poznámky
 export const notes = pgTable('notes', {
   id: uuid('id').primaryKey().defaultRandom(),