fix: propagate document_tags in async retain path

[abix5]

Summary

• fix async retain tag propagation by forwarding document_tags from the queued task payload in _handle_batch_retain to retain_batch_async(...)
• update MemoryEngineInterface signatures so async/sync batch retain methods explicitly accept optional document_tags
• add focused unit tests for async tag flow:
  • submit_async_retain includes document_tags in queued payload
  • worker handler forwards document_tags into retain execution

Why

When /memories was called with async=true and document_tags, tags were dropped in the worker path. As a result, tags did not reliably appear in /tags, and tag-scoped workflows (including scoped mental models) behaved incorrectly.

Testing

• /Users/dmitriynenashev/Projects/hindsight/.venv-hs/bin/python -m pytest tests/test_async_retain_tags.py -o addopts='' -q
• manual local API verification:
  • async retain with document_tags
  • verify /v1/default/banks/{bank_id}/tags contains the tag
  • verify tag-filtered recall returns expected results

Follow-up

There is a separate inconsistency in tags_match semantics across endpoints (recall/reflect vs mental-models/directives). This PR does not change that behavior; a dedicated follow-up PR will unify those semantics.

[nicoloboschi]

LGTM

Nice catch!