feat: 添加结构化日志与审计

- tracing + tracing-subscriber，全局 --verbose/-v 与 RUST_LOG 控制
- 新增 audit_log 表，add/update/delete 成功后自动写入审计记录
- 新增 src/audit.rs，审计失败仅 warn 不中断主流程
- 更新 README/AGENTS.md，补充 verbose、audit_log 说明
- .vscode/tasks.json 增加 verbose/update/audit 测试任务

Made-with: Cursor

src/audit.rs

@@ -0,0 +1,34 @@
+use anyhow::Result;
+use serde_json::Value;
+use sqlx::PgPool;
+
+/// Write an audit entry for a write operation. Failures are logged as warnings
+/// and do not interrupt the main flow.
+pub async fn log(
+    pool: &PgPool,
+    action: &str,
+    namespace: &str,
+    kind: &str,
+    name: &str,
+    detail: Value,
+) {
+    let actor = std::env::var("USER").unwrap_or_default();
+    let result: Result<_, sqlx::Error> = sqlx::query(
+        "INSERT INTO audit_log (action, namespace, kind, name, detail, actor) \
+         VALUES ($1, $2, $3, $4, $5, $6)",
+    )
+    .bind(action)
+    .bind(namespace)
+    .bind(kind)
+    .bind(name)
+    .bind(&detail)
+    .bind(&actor)
+    .execute(pool)
+    .await;
+
+    if let Err(e) = result {
+        tracing::warn!(error = %e, "failed to write audit log");
+    } else {
+        tracing::debug!(action, namespace, kind, name, actor, "audit logged");
+    }
+}