Add backup cleanup and retention policy controls

[paullizer]

Summary

Add admin-facing cleanup controls for Data Management backups, including manual backup deletion and an automatic retention policy that can delete backups after a configurable age expressed in days, weeks, months, or years.

User Value

Admins need a safe way to reduce backup storage growth and remove obsolete backups without editing Cosmos records or storage artifacts manually.

Proposed Behavior

• Admins can delete selected backups from Backup Inventory after confirmation.
• Data Management settings include a retention policy using a value and unit: days, weeks, months, or years.
• Retention cleanup removes expired backup metadata and associated stored backup artifacts consistently.
• Cleanup actions are logged as admin activity and return clear success/error feedback.

Acceptance Criteria

• Backup Inventory exposes safe manual delete controls for eligible completed/failed/canceled backups.
• Backend API validates and authorizes backup deletion requests for admins only.
• Retention policy settings support days, weeks, months, and years with bounded validation.
• Scheduled or background Data Management processing applies the retention policy without deleting backups newer than the cutoff.
• Functional and route-policy tests cover validation, authorization, retention cutoff calculation, and deletion behavior.
• UI tests cover retention settings and manual delete confirmation.

Notes

• Existing backup settings already include retention_days; this work should preserve compatibility while adding the requested unit-based policy.
• Relevant areas likely include functions_data_management.py, route_backend_data_management.py, admin_settings.html, admin_data_management.js, route tests, functional tests, and UI tests.

[paullizer]

Implemented in pull request #1135.