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/api/alert_rules.py

@@ -1136,7 +1136,8 @@ def run_ar_overdue_check(
         return payload
 
     if dry_run:
-        payload.update({"would_insert": 0 if blockers else len(hits), "write_blockers": blockers})
+        payload.update({"would_insert": 0 if blockers else ar_overdue_alert.count_would_insert(
+            db, entity_id, resolved_kpi_id, result), "write_blockers": blockers})
         return payload
 
     if blockers:
@@ -1147,6 +1148,6 @@ def run_ar_overdue_check(
                    "hit": len(hits), **written},
            entity_id=entity_id)
     db.commit()
-    payload.update({"would_insert": len(hits), "write_blockers": [], **written,
+    payload.update({"would_insert": written["inserted"], "write_blockers": [], **written,
                     "generated": written["inserted"]})
     return payload