feat: add segment saturation events

[itsDNNS]

Summary

• add persisted FRITZ!Box segment saturation event detection
• expose segment saturation events through the FRITZ!Box segment utilization API
• add a Segment Saturation Events widget with safe correlation handoff
• add regression coverage for event detection, downsampling persistence, API validation, and range handling

Test plan

• pytest tests/modules/test_fritzbox_cable_storage.py tests/modules/test_fritzbox_cable_routes.py -q
• node --check app/static/js/segment-utilization.js
• python3 scripts/i18n_check.py --validate
• pytest tests/e2e/test_segment_utilization.py -q --browser chromium
• pytest tests/e2e -q --browser chromium
• pytest -q --browser chromium

Closes #334