Deleting large Accounts/Inboxes with object.destroy! can time out and
create heavy destroy_async fan-out; this change adds a simple pre-purge
that batch-destroys heavy associations first .
```
Account: conversations, contacts
Inbox: conversations, contact_inboxes
```
We use find_in_batches(5000), then proceeds with destroy!, reducing DB
pressure and race conditions while preserving callbacks and leaving the
behavior for non heavy models unchanged. The change is also done in a
way to easily add additional objects or relations to the list.
fixes:
https://linear.app/chatwoot/issue/CW-3106/inbox-deletion-process-update-the-flow
