feat/prune age

[alanpeixinho]

Summary

Add prune_db, a manual management command to delete old data and reclaim space.

• Deletes checkouts, builds and tests older than a given age (e.g. --older-than "30 days").
• Manual cascade (models use DO_NOTHING): pruning a checkout removes its builds, pruning a build removes its tests — even when children are newer.
• --origins scopes the age filter; cascade stays origin-agnostic so no orphans are left.
• Batched deletes (--batch-size) that each commit on their own to keep locks short.
• --dry-run and a per-table confirmation prompt showing exactly how many rows will be affected (skip with --yes).
• Extracts parse_interval from update_db into a shared helper.

Usage

python manage.py prune_db --older-than "30 days" --dry-run                                                                                                                                                         
python manage.py prune_db --older-than "30 days" --origins maestro                                                                                                                                          

How to Test

• In a local database create newly seeded data poetry run python3 manage.py seed_data --clear --yes
• Run command poetry run python3 manage.py prune_db --dry-run with different arguments for older-than parameter.
• Verify the number of selected rows matches the database.
• Run command poetry run python3 manage.py prune_db with different arguments for older-than parameter.
• Verify the rows are properly deleted.