fix(ml): circular sleep std, HRV battery fallback, running economy thresholds, stress 75/25 blend

[gerfru]

Summary

• Sleep consistency: replace linear σ with circular Rayleigh R — eliminates midnight-wrap artifact (23:30 vs 00:30 was appearing as 23 h spread instead of 1 h)
• Body battery: HRV fallback now uses sleep_quality instead of fixed 0.5, removing the artificial [52.5, 88] score cap when no HRV data is available
• Running economy: unify GCT/VO/VR thresholds with the activity page UI (240 ms / 80 mm / 8 %) — they were inconsistent
• Stress score blend: rebalance HRV/Garmin weight to 75/25 (was 60/40); Garmin stress is itself HRV-derived, so over-weighting it double-counts the same signal
• Tests: circular midnight-wrap regression, stress blend ratio assertion, sleep consistency integration test

Test plan

• cd ml-service && .venv/bin/pytest tests/test_models.py -v — all new tests pass
• Sleep consistency score for users with ~midnight bedtime should now reflect actual regularity
• Body battery score without HRV data should track sleep quality instead of being capped at mid-range

🤖 Generated with Claude Code