tech-debt(backend): repoint 7 score→letter-grade forks onto canonical get_grade_from_score #12686

Round-2 of #12645. get_grade_from_score() (code_intelligence/shared/scoring.py:127, 90/80/70/60→A/B/C/D/F) is canonical but 7 sites redefine it byte-identically: api/analytics_code.py:555 _score_to_grade, api/code_intelligence.py:298 _calculate_grade_from_score, api/analytics_reporting.py:183 get_grade, plus api/anti_pattern.py, code_intelligence/performance_analysis/analyzer.py, code_intelligence/pattern_analysis/complexity_analyzer.py, services/analytics_service.py. Import the canonical, delete local defs. Refs #12645.