Fix dashboard SSE shutdown during autoupdate

[pilipilisbot]

Summary

• Signal dashboard SSE generators through the FastAPI lifespan shutdown event.
• Let job session streams exit while sleeping and close journal tail streams when shutdown begins.
• Document the shutdown behavior for dashboard autoupdates.

Fixes #171

Tests

• . .venv/bin/activate && pytest -q

Result: 341 passed, 1 existing Starlette/httpx deprecation warning.

[pilipilisbot]

Post-merge verification: PR #172 is merged into main as ba27a1ae54d1578fea53194a3e2f053f9152ebd6 (head 3b6dcc048fe1d02efd489183038484b63e9597ac). The linked issue #171 is closed, and the PR checks completed successfully: dashboard, pytest (3.11), and pytest (3.12). No follow-up code changes are needed for this merge event.