* Fix non-atomic async operation creation in _submit_async_operation
Previously the method performed two separate database round-trips:
1. INSERT into async_operations with no task_payload (null)
2. submit_task → UPDATE to set task_payload
A process crash or network error between steps 1 and 2 left a row with
task_payload IS NULL permanently. The worker's claim query requires
task_payload IS NOT NULL, so these orphaned rows could never be picked up
and the queue appeared degraded indefinitely.
Fix: build full_payload before the INSERT and include task_payload in the
same INSERT statement, making operation creation atomic. submit_task is
still called afterwards — for SyncTaskBackend it executes the task
immediately (unchanged behaviour); for BrokerTaskBackend it becomes an
idempotent UPDATE (payload already set) kept for symmetry.
* Preserve datetime payloads in atomic async insert
94cf89b570