Add test coverage for _validate_notification_target

[tmdeveloper007]

Problem

_validate_notification_target in backend/secuscan/routes.py handles webhook URL validation, SSRF protection, and email format validation. No direct unit tests exist.

Fix

1. Extract the function into backend/secuscan/routes_email_validation.py.
2. Add 19 unit tests covering valid/invalid webhook URLs, SSRF protection, email format, and empty/whitespace inputs.
3. Update routes.py to re-export from the new module.

Acceptance Criteria

• backend/secuscan/routes_email_validation.py created with extracted logic.
• testing/backend/unit/test_routes_email_validation.py with 19 tests passing locally.