v2.0.0: Multi-user authentication with JWT

Features:
- User registration and login with JWT tokens
- All data is now user-specific (multi-tenancy)
- Profile page with integrations and logout
- Automatic migration of existing data to first user

Backend changes:
- Added users and refresh_tokens tables
- Added user_id to all data tables (projects, entries, nodes, dictionaries, words, progress, configs, telegram_integrations, weekly_goals)
- JWT authentication middleware
- claimOrphanedData() for data migration

Frontend changes:
- AuthContext for state management
- Login/Register forms
- Profile page (replaced Integrations)
- All API calls use authFetch with Bearer token

Migration notes:
- On first deploy, backend automatically adds user_id columns
- First user to login claims all existing data

env.example

@@ -47,6 +47,15 @@ WEBHOOK_BASE_URL=https://your-domain.com
 # Оставьте пустым, если не хотите использовать проверку секрета
 TODOIST_WEBHOOK_SECRET=
 
+# ============================================
+# Authentication Configuration
+# ============================================
+# Секретный ключ для подписи JWT токенов
+# ВАЖНО: Обязательно задайте свой уникальный секретный ключ для production!
+# Если не задан, будет использован случайно сгенерированный (не рекомендуется для production)
+# Можно сгенерировать с помощью: openssl rand -base64 32
+JWT_SECRET=your-super-secret-jwt-key-change-in-production
+
 # ============================================
 # Scheduler Configuration
 # ============================================