feat: 密钥静态加密 + /api/v1 agent 管理 API

密钥加密（写侧封存 / 读侧解封）
- config.yaml 的 sources[].api_key、sources[].headers、keys[].key 落盘即
  AES-256-GCM 密文（enc:v1: 前缀），master.key 复用 runtime store 那把
- 内存里永远是明文：鉴权比对、API 返回新建 key、WebUI 编辑回填都不受影响
- 启动时一次性封存现存明文（幂等，已封存则不写盘）；-check 不写文件
- UpsertSourceInYAML 增加 box 参数，新加的源不再以明文落盘
- 解密失败改为硬错误：原先 MustDecrypt 返回密文会被下次 Save 二次封存
  （实测：源 key 18→20、静默损坏），现在启动即失败且配置分毫不动

/api/v1：面向 agent 的管理 API（WebUI 零影响）
- GET /api/v1            机器可读索引，列出每个端点的方法/权限/用途
- GET /api/v1/overview   一次调用看全貌：源 + AUTO 链 + 密钥数 + 健康度
- GET /api/v1/health     仅健康快照
- GET /api/v1/models     按源分组的可路由模型清单
- GET /api/v1/sources[/{name}]  凭据遮蔽后的源
- GET /api/v1/auto       调度链与实时槽位状态
- GET /api/v1/keys       admin only，密钥元数据，绝不回显密钥本身
- 沿用同一套网关 key 鉴权；读端点任意角色，写仍需 admin

测试：15 个新用例（含负向：泄密、越权、写操作必须被拒）
变异验证：maskKey 不遮蔽→红、去掉 admin 校验→红

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -27,6 +27,11 @@ type Config struct {
 	Auto          []ModelScope `yaml:"auto,omitempty"`       // AUTO 调度链规则（WebUI 优先级页编辑，chat）
 	AutoImage     []ModelScope `yaml:"auto_image,omitempty"` // AUTO 生图调度链规则（WebUI 优先级页·生图）
 	Keys          []GWKey      `yaml:"keys,omitempty"`       // 网关密钥（WebUI 密钥页管理）
+	// box seals credentials (sources' api_key/headers, keys' key) at rest.
+	// In-memory values are always plaintext; only the bytes on disk are sealed.
+	// Wired by AttachSecretBox — Load leaves it nil so `-check` and tests stay
+	// filesystem-free.
+	box *SecretBox
 }
 
 // Defaults applied to any source (YAML or runtime) that leaves a field unset.
@@ -192,10 +197,15 @@ func RemoveSourceFromYAML(path, name string) error {
 
 // UpsertSourceInYAML adds or updates a source entry in the YAML config file.
 // Uses yaml.Node to preserve the rest of the file's comments and formatting.
-func UpsertSourceInYAML(path, name string, src Source) error {
+// When box is non-nil the source's credentials are sealed before writing, so a
+// newly added source never lands in the file as plaintext.
+func UpsertSourceInYAML(path, name string, src Source, box *SecretBox) error {
 	if path == "" {
 		return fmt.Errorf("config path is empty")
 	}
+	if err := sealSource(&src, box); err != nil {
+		return err
+	}
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return err
@@ -253,11 +263,17 @@ func UpsertSourceInYAML(path, name string, src Source) error {
 
 // Save writes the current config back to the YAML file (preserving comments
 // via yaml.Node round-trip when possible, or full marshaling as fallback).
+// Credentials are sealed on the way out and the in-memory copy is restored to
+// plaintext afterwards, so callers keep working with usable values.
 func (c *Config) Save() error {
 	if c.Path == "" {
 		return fmt.Errorf("config path is empty")
 	}
+	if err := c.sealInPlace(c.box); err != nil {
+		return err
+	}
 	out, err := yaml.Marshal(c)
+	c.unsealAfterWrite(c.box)
 	if err != nil {
 		return fmt.Errorf("marshal config: %w", err)
 	}