When a user is deleted from the panel, the subscription may already be
cascade-deleted by the time the webhook handler tries to decrement
server counters. This caused StaleDataError followed by
PendingRollbackError when accessing subscription.id in the error handler.
- Save subscription.id before DB operations to avoid lazy load after rollback
- Catch StaleDataError explicitly and rollback the session
- Re-fetch subscription/user after potential rollback in _handle_user_deleted
- Skip subscription cleanup if it was already cascade-deleted
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