Align OFDM event and report thresholds

[itsDNNS]

Summary

• use analyzer channel health for family-tagged SNR/MER event transitions so good-range OFDM changes do not fall back to SC-QAM thresholds
• use family-aware downstream power and SNR/MER thresholds in report diagnostics
• show the threshold for the channel that supplied the historical downstream minimum, while preserving the legacy fallback when channel evidence is unavailable

Related to #743.

Testing

• python -m pytest -q tests/test_events.py tests/web/test_reports.py tests/analyzer tests/test_docsis_case_fixtures.py: 300 passed
• python -m pytest -q tests --ignore=tests/e2e: 2965 passed, 1 skipped
• browser E2E by file: 502 passed; test_correlation_hover_shows_tooltip_and_highlights_timeline fails locally on both this branch and unchanged origin/main