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

Cargo.toml

@@ -13,4 +13,6 @@ serde_json = "1.0.149"
 sqlx = { version = "0.8.6", features = ["runtime-tokio", "tls-rustls", "postgres", "uuid", "json", "chrono"] }
 tokio = { version = "1.50.0", features = ["full"] }
 toml = "1.0.7"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 uuid = { version = "1.22.0", features = ["serde", "v4"] }