Add retention Discord User-Agent

[cmyui]

Summary

• add an explicit User-Agent header to the retention Discord webhook request
• assert the header in the webhook unit test

Context

The manual retention run succeeded, but Discord returned HTTP Error 403: Forbidden for the Python urllib request. The backup job uses curl, which sends a User-Agent by default; this makes the retention webhook request behave similarly.

Verification

• ./venv/bin/python -m unittest -v
• pre-commit run --files backup_retention_policy.py test_backup_retention_policy.py
• git diff --check