[Infra] Add deterministic readiness invariant tests

[shchukins]

Summary

• extend readiness invariant coverage with deterministic monotonicity checks across load, recovery, and readiness
• add regression-style tests for missing-HRV recovery days and numerically valid fallback outputs
• keep the change local to backend/tests/test_readiness_invariants.py with concise comments explaining why each invariant matters

Tests

• PYTHONPATH=. ./venv/bin/pytest tests/test_readiness_invariants.py tests/test_readiness_fallbacks.py tests/test_load_state_v2.py tests/test_readiness_query.py

Assumptions

• current readiness baseline remains 0.6 * freshness_norm + 0.4 * recovery_score_simple
• missing HRV should degrade gracefully by using neutral HRV contribution rather than failing the recovery pipeline
• fallback modes (full, load_only, recovery_only) are part of the supported deterministic contract

Risks / Gaps

• coverage is focused on service-level invariants and does not add new DB integration fixtures
• no_data still remains covered via existing 404 behavior tests rather than a new end-to-end pipeline test

Refs #99