Background tasks (async retain, consolidation, reflections) fail in
multi-tenant deployments because the worker executes tasks without
setting the tenant schema context. This causes two failures:
1. The cancellation check in execute_task queries public.async_operations
instead of the tenant's schema, finds no row, and skips the task as
"cancelled" — even though it wasn't.
2. Even if that were fixed, _authenticate_tenant would throw
AuthenticationError because background tasks have no API key.
Changes:
- Poller passes task.schema into task_dict so execute_task can set it
- execute_task sets _current_schema before the cancellation check
- Task handlers use RequestContext(internal=True) to signal background ops
- _authenticate_tenant skips extension auth for internal requests when
schema is already set
- BrokerTaskBackend uses schema_getter for dynamic schema resolution
when submitting tasks and waiting for results
- Pass tenant_extension to WorkerPoller in create_app
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