feat(db): 添加 ai_suggestion 和 ai_risk_threshold 表迁移

- ai_suggestion: AI 建议记录表，含 tenant_id、analysis_id、suggestion_type、
  risk_level、status、params、baseline_snapshot 等字段
- ai_risk_threshold: 租户级风险阈值配置表，按 metric_name + tenant_id 唯一索引
- 两表均包含标准审计字段和 version_lock 乐观锁

crates/erp-server/migration/src/lib.rs

@@ -97,6 +97,8 @@ mod m20260429_000094_device_readings_unique_constraint;
 mod m20260429_000095_seed_alert_device_menus;
 mod m20260430_000096_create_medication_reminder;
 mod m20260501_000097_seed_menu_permissions;
+mod m20260501_000098_create_ai_suggestion;
+mod m20260501_000099_create_ai_risk_threshold;
 
 pub struct Migrator;
 
@@ -201,6 +203,8 @@ impl MigratorTrait for Migrator {
             Box::new(m20260429_000095_seed_alert_device_menus::Migration),
             Box::new(m20260430_000096_create_medication_reminder::Migration),
             Box::new(m20260501_000097_seed_menu_permissions::Migration),
+            Box::new(m20260501_000098_create_ai_suggestion::Migration),
+            Box::new(m20260501_000099_create_ai_risk_threshold::Migration),
         ]
     }
 }