feat: Add comprehensive auto-sync functionality

✨ New Features:
- Auto-sync service with configurable intervals (15min, 30min, 1hour, 6hours, 12hours, 24hours, custom cron)
- Automatic JSON file synchronization from GitHub repositories
- Auto-download new scripts when JSON files are updated
- Auto-update existing scripts when newer versions are available
- Apprise notification service integration for sync status updates
- Comprehensive error handling and logging

🔧 Technical Implementation:
- AutoSyncService: Core scheduling and execution logic
- GitHubJsonService: Handles JSON file synchronization from GitHub
- AppriseService: Sends notifications via multiple channels (Discord, Telegram, Email, Slack, etc.)
- ScriptDownloaderService: Manages automatic script downloads and updates
- Settings API: RESTful endpoints for auto-sync configuration
- UI Integration: Settings modal with auto-sync configuration options

📋 Configuration Options:
- Enable/disable auto-sync functionality
- Flexible scheduling (predefined intervals or custom cron expressions)
- Selective script processing (new downloads, updates, or both)
- Notification settings with multiple Apprise URL support
- Environment-based configuration with .env file persistence

🎯 Benefits:
- Keeps script repository automatically synchronized
- Reduces manual maintenance overhead
- Provides real-time notifications of sync status
- Supports multiple notification channels
- Configurable to match different deployment needs

This feature significantly enhances the automation capabilities of PVE Scripts Local,
making it a truly hands-off solution for script management.

Author: michelroegl-brunner