Add decision rules and backend recommendation layer

[shchukins]

Decision layer v1

Implemented deterministic readiness → recommendation mapping.

Done

• documented decision rules v1
• added backend recommendation layer
• readiness API now returns:
  • recommendation
  • reason
• added rule-based mapping:
  • < 40 → recovery
  • 40–60 → endurance
  • 60–75 → moderate
  • >= 75 → high intensity possible
• kept logic deterministic and explainable
• updated docs/tests

Example

{
  "recommendation": "endurance",
  "reason": "Readiness score is 55.7/100. Freshness is available at 52.7/100. Recovery is available at 60.1/100. Recommendation is endurance."
}

Closes #46
Closes #47