feat(saas): add down migrations for all incremental schema changes (AUD3-DB-01)

- 16 down SQL files in migrations/down/ for each incremental migration
- db::run_down_migrations() executes rollback files in reverse order
- migrate_down CLI task: task=migrate_down timestamp=20260402
- Initial schema and seed data excluded (would be destructive)

docs/features/AUDIT_TRACKER.md

@@ -228,7 +228,7 @@
 | AUD3-FE-04 | window 全局变量存 interval | **FALSE_POSITIVE** | App.tsx 已使用 useRef，非 window 全局变量 |
 | AUD3-FE-05 | 25+ 处 mixin `as any` | **FIXED** | gateway-heartbeat.ts 删除(死代码) + kernel-*.ts 改为 Record<string, unknown> + chatStore 改为 Record; as any 38→3 |
 | AUD3-FE-06 | PropertyPanel 17 处 `as any` | **FIXED** | 13 处 (data as any) → 类型安全 d 访问器 (Record<string, unknown>) |
-| AUD3-DB-01 | 无 down migration | OPEN | crates/zclaw-saas/migrations/ |
+| AUD3-DB-01 | 无 down migration | **FIXED** | 16 个 down SQL + run_down_migrations() + migrate_down task |
 | AUD3-DB-02 | format! SQL 模式 | **FALSE_POSITIVE** | 同 SEC2-P2-08，白名单+防御性验证已到位 |
 | AUD3-API-02 | 前端错误处理不统一 | **DOCUMENTED** | error-handling.ts 基础设施已完善，100+ 文件渐进式迁移至 reportError/reportApiError |
 | AUD3-CONC-02 | ~15 处 fire-and-forget tokio::spawn | **FALSE_POSITIVE** | 与 SEC2-P2-05 相同，已添加 NOTE(fire-and-forget) 注释 |
@@ -273,6 +273,7 @@
 | 2026-04-05 | SEC2-P3-02 | OPEN → DOCUMENTED | P4 级差异，admin 独立类型系统 |
 | 2026-04-05 | V11-P2-05 | 部分关闭 → FIXED | 完整审计: 160 @connected + 16 @reserved + 1 未注册 = 177 总命令 |
 | 2026-04-05 | TYPE-01 | OPEN → FIXED | ConnectionState 统一(含 handshaking); saas-types PromptTemplate/PromptVariable union literal; CreateRoleRequest optional; PropertyPanel as any→类型安全; chatStore window cast |
+| 2026-04-05 | AUD3-DB-01 | OPEN → FIXED | 16 个 down migration SQL + db::run_down_migrations() + migrate_down task (timestamp=20260402) |
 
 ## V12 模块化端到端审计修复 (2026-04-04)