Implement Apprise notifications system

[Copilot]

This PR implements a comprehensive notification system using the Apprise library which supports over 65 different notification services through a single unified API.

Features added:

• Added Apprise dependency to requirements.txt
• Created a dedicated notifications module in src/primary/utils/notifications.py
• Added notification settings to the General Settings UI section:
  • Enable/disable notifications toggle
  • Notification level selection (debug, info, warning, error)
  • Apprise URL configuration field
  • Test notification button
• Integrated notifications with key application events:
  • Hunt start/stop notifications
  • Support for future integrations

How to use:

1. Go to the Settings section in Huntarr
2. Configure your preferred notification service with an Apprise URL:
   • Discord: discord://webhook_id/webhook_token
   • Telegram: tgram://bottoken/ChatID
   • Pushover: pover://user@token
   • Many more options available in the Apprise documentation
3. Toggle "Enable Notifications" to activate the system
4. Use the "Test Notification" button to verify your configuration

Screenshot

(No screenshots available in this environment, but the UI changes are minimal and fit seamlessly with the existing Settings page design)

Fixes #257.

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