feat(notifications): Add Slack Webhook Integration for Scan Completion Alerts

[Rafiaminhaj]

Description

This PR implements a client-side Slack Webhook integration to post structured block alerts to a configured Slack channel when a security scan completes or fails.

• Added slack_webhook_url configuration, customizable via SECUSCAN_SLACK_WEBHOOK_URL env variable.
• Created process_slack_notification handler in notification_service.py to:
  • Extract scan status, target, duration, and findings counts.
  • Construct a clean Slack Block Kit layout detailing severity breakdowns (Critical, High, Medium, Low, Info) or failed error messages.
  • Deliver notifications securely using SSRF/DNS-rebinding protected webhooks.
• Integrated the handler into the executor pipeline so it fires automatically at the end of task execution.

Related Issues

Closes #1133

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Added extensive backend unit tests: test_process_slack_notification_success and test_process_slack_notification_failed_task inside test_notification_service.py.
• Ran the unit test suite locally using pytest:

  venv\Scripts\python -m pytest testing/backend/unit/test_notification_service.py
  # Result: 22 passed successfully!