fix(alerts): kpi_alerts 去重键改生成侧语义键 dedupe_key（A2）— 文案改动不再重复落库

- 列 dedupe_key VARCHAR(160) NULL + 唯一索引 (entity_id,kpi_id,alert_type,dedupe_key)
- write_alerts 三级判重：dedupe_key 命中 / 历史 NULL 行反解兼容 / 无命中插入
- 并发兜底：savepoint 捕获 IntegrityError 计跳过
- dry-run would_insert 改用 count_would_insert（修复 len(hits) 虚高）
- DDL 幂等执行器 scripts/ddl_kpi_alerts_dedupe.py（check/apply/rollback）
- tests/test_dedupe_key.py 7 passed（T1-T7b）；alert/overdue 回归 173 passed

backend/app/models/__init__.py

@@ -167,6 +167,8 @@ class KPIAlert(Base):
     alert_level = Column(String(20), default="yellow", comment="green/yellow/red")
     alert_message = Column(String(500), nullable=False)
     alert_type = Column(String(30), default="actual", comment="actual/forecast — 实际值超限/预测值超限")
+    dedupe_key = Column(String(160), nullable=True,
+                        comment="生成侧幂等键 period|scope|ident；仅 alert_type=overdue 填充，历史行为 NULL")
     suggestion = Column(Text, nullable=True, comment="情景建议")
     action_plan_linked_id = Column(Integer, nullable=True, comment="关联的改善计划ID")
     status = Column(String(20), default="pending", comment="pending/processing/resolved")