refactor(audit): 移除旧格式兼容，user_id 统一走列字段

- audit_log 查询去掉 detail->>'user_id' 回退分支
- login_detail 不再冗余写入 user_id 到 detail JSON
- 迁移 SQL 去掉多余的 ALTER TABLE ADD COLUMN

Made-with: Cursor

crates/secrets-core/src/db.rs

@@ -77,7 +77,6 @@ pub async fn migrate(pool: &PgPool) -> Result<()> {
             created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW()
         );
 
-        ALTER TABLE audit_log ADD COLUMN IF NOT EXISTS user_id UUID;
         CREATE INDEX IF NOT EXISTS idx_audit_log_created  ON audit_log(created_at DESC);
         CREATE INDEX IF NOT EXISTS idx_audit_log_ns_kind  ON audit_log(namespace, kind);
         CREATE INDEX IF NOT EXISTS idx_audit_log_user_id  ON audit_log(user_id) WHERE user_id IS NOT NULL;