fix: cancel journal read before closing stream

[giscebot]

Summary

• cancel and await pending journal read/shutdown tasks before closing the async generator
• prevent client disconnect cancellation from racing with aclose()
• add a regression test that reproduces the Sentry failure

Tests

• pytest -q tests/test_backend.py::test_dashboard_journal_stream_cancels_pending_read_when_client_disconnects tests/test_backend.py::test_dashboard_journal_stream_exits_when_shutdown_is_signaled (2 passed)
• full suite: 342 passed when deployment-specific dashboard/GitHub environment variables are unset; the initial inherited-environment run exposed only environment-dependent pre-existing failures

Fixes gisce#28

Requested by: automated Sentry alert from gisce-ti-sentry[bot] (no human requester)

[pilipilisbot]

Approved. I checked the linked Sentry issue, the PR diff around _journal_stream_events, the stream_journal_lines cleanup path, whitespace with git diff --check, the focused regression tests locally in a temporary venv, and the PR CI checks.

The change cancels and awaits the outstanding journal read/shutdown tasks before calling aclose(), which addresses the async-generator-running race seen when Starlette cancels the SSE response. The regression coverage exercises both dashboard shutdown and client-disconnect cancellation, and I do not see a correctness or operational blocker.

Local verification: /tmp/gab-pr175-venv/bin/python -m pytest -q tests/test_backend.py::test_dashboard_journal_stream_cancels_pending_read_when_client_disconnects tests/test_backend.py::test_dashboard_journal_stream_exits_when_shutdown_is_signaled passed: 2 passed, 1 Starlette/httpx deprecation warning.

GitHub checks: dashboard, pytest (3.11), and pytest (3.12) are passing.