fix: prevent duplicate SSE events at connect

[Iams4kura]

Summary

• Fixes: An event emitted after /chat/events subscribes but before its StreamingResponse body starts is sent once as replay and again as live, producing duplicate conversation rows.
• Root cause: The listener queue is registered when the endpoint runs, but the history snapshot is delayed until the response generator first iterates, so an intervening event enters both history and the already-subscribed queue.

Regression evidence

• Before: .venv/bin/python -m pytest -q tests/test_web_service.py::test_an_event_after_subscription_is_delivered_once_as_live exited 1

• After: .venv/bin/python -m pytest -q tests/test_web_service.py::test_an_event_after_subscription_is_delivered_once_as_live exited 0

Verification

• .venv/bin/python -m pytest -q tests/test_web_service.py
• .venv/bin/python -m pytest -q
• .venv/bin/python scripts/check_english_only.py --selftest && .venv/bin/python scripts/check_english_only.py
• .venv/bin/python scripts/check_content.py --selftest && .venv/bin/python scripts/check_content.py
• AIHAWK_CHECK_VERSION=1 .venv/bin/python -m pytest -q tests/test_version_is_not_taken.py
• .venv/bin/python -m build --wheel; install the wheel in a clean virtual environment; run aihawk --help and import shipped modules

Scope

• 3 files changed, +38 / -2 lines