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/migrations/0006_add_todo_users_table.sql

@@ -0,0 +1,29 @@
+-- Migration: Add todo_users junction table and migrate from assignedTo
+-- Created: 2025-11-24
+-- Description: Allows many-to-many relationship between todos and users
+
+-- Create todo_users junction table
+CREATE TABLE IF NOT EXISTS todo_users (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    todo_id UUID NOT NULL REFERENCES todos(id) ON DELETE CASCADE,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    assigned_by UUID REFERENCES users(id) ON DELETE SET NULL,
+    assigned_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    CONSTRAINT todo_user_unique UNIQUE(todo_id, user_id)
+);
+
+-- Create indexes for better query performance
+CREATE INDEX IF NOT EXISTS idx_todo_users_todo_id ON todo_users(todo_id);
+CREATE INDEX IF NOT EXISTS idx_todo_users_user_id ON todo_users(user_id);
+
+-- Migrate existing assignedTo data to todo_users table
+INSERT INTO todo_users (todo_id, user_id, assigned_by, assigned_at)
+SELECT id, assigned_to, created_by, created_at
+FROM todos
+WHERE assigned_to IS NOT NULL;
+
+-- Drop the old assigned_to column
+ALTER TABLE todos DROP COLUMN IF EXISTS assigned_to;
+
+-- Add comment
+COMMENT ON TABLE todo_users IS 'Junction table for many-to-many relationship between todos and users (assigned users)';

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(),