refactor(worker): delete each workspace as soon as its teardown finishes

[viethung0o0]

Summary

Follow-up to #931. During the scheduled workspace purge, each workspace row is now removed as soon as its own teardown finishes, instead of waiting to remove them all together at the end of the batch.

Behavior

• A workspace is deleted immediately after it is torn down cleanly.
• A workspace whose teardown fails keeps its row and is retried on the next run.
• Deletions run with bounded concurrency alongside the teardown work.

Test plan

• business workspace service tests (per-workspace deletion covered)
• type-check on the changed file
• lint on the changed files

Note: the repo-wide type-check hook currently fails on unrelated, pre-existing errors in other packages; the file changed here type-checks clean.