Add notification system with NTFY support

[kCn3333]

feat: Add notification system with NTFY support

Implements flexible notification system for backup completion/failure alerts.
Supports multiple notification providers with easy extensibility.

Features:

• NTFY integration with JSON API
• URL format (scheme://user:pass@host/topic?params)
• HTTP Basic Authentication support
• HTTPS/HTTP configurable via query param
• Emoji tags for visual categorization
• Markdown-formatted messages
• Per-backup notification control (notifyOnComplete flag)
• Multiple notification services simultaneously
• Graceful handling of missing/empty configuration
• Async delivery with fail-safe error handling

Configuration:

• notification.urls: List of notification URLs (optional)
• notification.timeoutSeconds: HTTP timeout (default: 10)

Database:

• Added notify_on_complete column to backup_configurations
• Migration: V3__add_notify_on_complete.sql
• Default: false (backward compatible)

Architecture:

• NotificationService interface for extensibility
• NtfyNotificationService implementation
• NotificationListener for event handling
• NotificationUrlParser for Shoutrrr format
• NotificationMessageBuilder for formatting
• RestTemplate-based HTTP client

Future-ready for:

• Webhook support
• Discord integration
• Slack integration
• Email notifications