merge: code-review fixes (d7720662 baseline + 9f8a 文案常量化、env_prefix 测试、补充用例); secrets-mcp 0.5.21

- default 已 rebase 到 d7720662；合并说明见 plans/merge-code-review-fixes-2026-04-11.md
- Web PATCH 长度错误用 validation 常量拼接；env_map 单测；import/api_key 单测
- rustfmt 收尾

Cargo.lock

@@ -2065,7 +2065,7 @@ dependencies = [
 
 [[package]]
 name = "secrets-mcp"
-version = "0.5.20"
+version = "0.5.21"
 dependencies = [
  "anyhow",
  "askama",