fix: avoid MissingGreenlet during OKR focus upsert

[yaojin3616]

Summary

• Refresh focus items after flushing an outer async session and before serializing them.
• Add regression coverage for the flush/refresh ordering.

Root cause

_upsert_focus_item_impl() serialized created_at and updated_at after flush() without loading server-generated values. With the asyncpg-backed SQLAlchemy session used by OKR startup seeding, that could cause an implicit lazy load outside the greenlet context and raise MissingGreenlet.

Validation

• PYTHONPATH=. uv run pytest tests/test_agent_seeder_storage_repair.py tests/test_focus_service.py
• PYTHONPATH=. uv run ruff check app/services/focus_service.py tests/test_focus_service.py

Fixes #813