fix(worker): quarantine invalid async operations

[ai-ag2026]

Summary

• quarantine pending async operations that can never be claimed because task_payload is NULL
• preserve legitimate batch_retain parent rows while they still have child operations
• skip the PostgreSQL JSONB quarantine query entirely on non-PostgreSQL backends

Why

Crash recovery can leave orphaned batch_retain parent rows in async_operations with status='pending' and task_payload=NULL. The worker claim path intentionally ignores NULL payloads because there is no executable task to dispatch, so these rows can sit in the pending lane forever and make queue health look like normal backlog.

Test Plan

• uv run ruff check hindsight_api/worker/poller.py tests/test_worker_quarantine.py
• uv run python -m py_compile hindsight_api/worker/poller.py tests/test_worker_quarantine.py
• git diff --check
• uv run --extra embedded-db pytest tests/test_worker_quarantine.py -q -o addopts=
• uv run --extra embedded-db pytest tests/test_worker_quarantine.py tests/test_worker.py -q -o addopts= -k 'not memory_engine_execute_task_passes_through_defer_operation' -> 67 passed, 1 deselected

Fixes #1671
Complements #1670 by handling the queued poison rows separately from embedding-dimension validation.