Add backup cleanup and retention policy controls

[paullizer]

Summary

• Add unit-based backup retention settings for days, weeks, months, and years while preserving retention_days compatibility.
• Add admin backup cleanup APIs for manual deletion and retention cleanup, including blob artifact deletion, job item cleanup, and latest-item sidecar invalidation.
• Add Backup Inventory delete controls, a retention cleanup action, and confirmation UX in Admin Settings.

Validation

• python -m pytest functional_tests\test_data_management_backup_durability.py ui_tests\test_admin_data_management_settings_ui.py
• python functional_tests\route_tests\test_route_blueprint_policy_inventory.py
• python functional_tests\route_tests\test_route_unauthenticated_policy_contract.py
• python functional_tests\route_tests\test_route_policy_test_coverage.py
• python -m py_compile application\single_app\functions_data_management.py application\single_app\route_backend_data_management.py functional_tests\test_data_management_backup_durability.py ui_tests\test_admin_data_management_settings_ui.py
• node --check application\single_app\static\js\admin\admin_data_management.js
• git diff --check

Closes #1130